Cu layers grown on Si(111) substrates by thermal inducted chemical vapor deposition

Surface & Coatings Technology 201 (2007) 9015 – 9020 www.elsevier.com/locate/surfcoat

Ag/Cu layers grown on Si(111) substrates by thermal inducted chemical vapor deposition I.B. Szymańska a,⁎, P. Piszczek a , W. Bała b , K. Bartkiewicz b , E. Szłyk a a

b

Faculty of Chemistry, Nicolaus Copernicus University, ul. Gagarina 7, 87-100 Toruń, Poland Department of Physics, Nicolaus Copernicus University, ul. Grudziądzka 3, 87-100 Toruń, Poland Available online 3 May 2007

Abstract Silver/copper thin layers were deposited on Si(111) substrates by thermal inducted chemical vapor deposition (CVD) method using [Cu (OOCC2F5)(VTMS)] (1), [Ag(OOCC2F5)] (2), and [Ag(OOCBut)(PEt3)] (3) as precursors. Analysis of grazing incidence X-ray diffraction (GIXRD) data confirms formation of silver/copper materials. The morphology studies, by scanning electron microscopy (SEM), exhibited that the type of Ag precursor, Ag(I)/Cu(I) precursors weight ratio, and the stability of metal containing species transported in vapors are the main factors, which influence the structure, size, and packed density of Ag/Cu layers. Depending on the weight ratio of precursors [Cu(OOCC2F5)(VTMS)] and [Ag(OOCBut)(PEt3)] the different type of silver/copper materials were prepared: the silver dispersed grains covered by the copper film composed of packed-density grains; the layer composes of Ag and Cu dense-packed grains; the copper layer consisted of large grains covered by silver one and the silver membrane containing copper grains located on the film surface. The low level of impurities adsorbed on the obtained surfaces was confirmed by diffuse reflectance FT IR (DRIFT) spectra. The investigations of electrical properties for fabricated Ag/Cu layers have been carried out using four-point probe technique. The highest conductivity [∼ 5.99 · 104 (Ω m)− 1] of films composed from Ag grain chains linked with the single Cu grains was noticed. © 2007 Elsevier B.V. All rights reserved. Keywords: Chemical vapor deposition; Ag/Cu layers; Ag(I) precursor; Cu(I) precursor; SEM; Electrical properties

1. Introduction Copper composites material such as: Cu/Ag, Cu/Pd and Cu/ Sn can be alternative materials for interconnections in microelectronic circuits due to improved migration resistance without increasing of electrical resistivity [1,2]. Moreover the metallic nanostructures are interesting, since it has been found that multilayers composed of alternating ferromagnetic and nonmagnetic metals reveal an oscillatory magnetic coupling and a giant magnetoresistance. Non-destructive pulsed high field magnets are powerful tools for the research at increasingly high magnetic fields (80–100 T). Among others Cu–Ag microcomposites are promising materials for the application in the mentioned magnets [3–5]. Chemical Vapor Deposition (CVD) is an alternative method of growing high purity films, particularly relevant to the ⁎ Corresponding author. Fax: +48 56 652 477. E-mail address: [email protected] (I.B. Szymańska). 0257-8972/$ - see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.surfcoat.2007.04.111

metallization of microelectronic and optic devices. The major problem in CVD of silver and copper composites is selection of stable and volatile precursors. Presently, complexes of fluorinated β-diketonates [M(β-diketonate)L], where M = Cu, Ag, L = PR3 (R = Me, Et), silanes (e.g. VTES = vinyltrimethylsilane), or diamines (e.g. tetramethylethylenediamine), are applied as CVD precursors [1]. The majority of these complexes are moisture or light sensitive, do not sublime easily (due to decomposition) and require special vaporization techniques. Therefore we have focused our research on fluorinated and nonfluorinated Ag(I), Cu(I) carboxylate complexes, of general formula [M(OOCR)(L)], where M = Ag, Cu; R = CF3, C2F5, C6F13; But, L = PR3, P(OR), which can be used as CVD precursors [6]. In the presented paper, the results of silver-copper CVD experiments, with [Cu(OOCC2F5)(VTMS)] VTMS = vinyltrimethylsilane (1) and [Ag(OOCC2F5)] (2) and [Ag(OOCBut) (PEt3)] (3) will be discussed. Moreover, the relations between the morphology of films, obtained at different conditions, and

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Table 1 The main CVD conditions for experiments with application 1–3 as precursors Precursor

Vaporization temperature (TV) [K] Substrate temperature (TD) [K] Total reactor pressure (p) [Pa] Deposition time [min] Precursor mass [mg]

1

3

1/2

1/3

Solid Solid

Oil

Mixture

Solid Oil

513

513

433

513

513

433

713

633–693 493–553 633–713 713

523

100

150

150

150

150

50 100

60 100

60 100

90 80:80

2 × 40 100:100 100:200 100:150 150:100

Carrier gas Substrates

2

Ar Si(111)

their electrical conductivity as a function of temperature, are described. 2. Experimental Syntheses of [Cu(OOCC2F5)(VTMS)] (1), [Ag(OOCC2F5)] (2) and [Ag(OOCBut)(PEt3)] (3) were carried out as reported [12,13,7]. Infrared (IR) spectra were recorded on a PerkinElmer Spectrum 2000 spectrometer (400–4000 cm 1 , resolution 4.0 cm− 1). Temperature variable IR (VT-IR) (303–523 K) spectra were measured with a SPECAC variable temperature cell, at dynamic vacuum p = 10− 2 mbar on the same instrument. The deposition experiments were carried out in horizontal hot-wall CVD reactor as described [8,9]. Silver/copper films were deposited on Si(111) substrates, under 150 Pa, in the deposition temperature (TD) range 523–713 K. Ag/Cu films were characterized by X-ray diffraction (XRD). GIXRD patterns were collected with a Philips X'PERT diffractometer. Diffractograms were registered in 20–80° 2θ ranges, using CuKα irradiation and sample spinning (receiving slit — 0.5, incidence angle — 0.5°, measuring time — 11 s per point). Moreover the purity of fabricated films surface was studied using DRIFT method (PerkinElmer DRIFT equipment). Morphology studies were carried out using a scanning electron microscopy (SEM-LEO 1460V, operating voltage 28 kV, the Secondary Electrons (SE) and Backscattered Electrons (BSE) detectors). The four-point probe technique has been used in order to determine Ag/Cu films electrical properties in the temperature range 103–333 K. The conductivity of the samples was taken as the inverse of the resistivity (ρ), which was determined with applied currents of 1 to 50 mA.

(OOCC2F5)(VTMS)] (1) in thermal inducted CVD process (Table 1) leads to formation good quality copper films. For that reason 1 was applied in all Ag/Cu CVD experiments. The thermolysis of 1 proceeds in two stages, starting from detachment of vinyltrimethylsilane between 353 and 433 K, followed by the formation of the stable copper(I) pentafluoropropionate in the solid state (433–453 K). The next, volatile and thermally stable copper binding species arise by copper(I) carboxylate decomposition in the range 453–513 K (Fig. 1). [Ag(OOCC2F5)] (2), and [Ag(OOCBut)(PEt3)] (3) were selected as silver precursors, which can be supplied together with 1 (Table 1). The previous reports [11–13] demonstrate that 2 and 3 precursors form Ag films with low contents of impurities. The thermolysis of 2 leads to the formation of volatile silver(I) carboxylate species between 493 and 513 K. Their stability was suitable to their transport in the gas phase [12]. The partial decomposition of 3, was observed during the vaporization. VT IR studies revealed the stable silver– phosphine species between 413 and 433 K (Fig. 1) [13]. Deposition experiments were carried out in a horizontal hotwall CVD reactor that was equipped for supply two precursors simultaneously. Thermal and volatile properties of silver and copper precursors determined the way of their delivery to the reactor. Because 1 and 2 reveal similar vaporization temperatures and same type of metal carriers (metal carboxylates), hence they were used as a mixture of precursors in the solid phase. In the second system (1 and 3) was a significant difference in vaporization temperatures of precursors and other types of copper and silver carriers (carboxylates for Cu; phosphines for Ag) therefore two vaporization vessels were applied in the precursor delivery system. According to data in Table 1, TV = 513 K have been proposed as an optimal vaporization temperature of 1 and 2 system. Therefore in CVD process, mixture of 1 and 2 (weight ratio 1:1) was heated to 513 K, keeping the constant reactor pressure and the carrier gas flow (Ar, 830 cm3/min). The Ag/Cu films were deposited between 663 and 683 K. XRD diffraction lines confirmed the presence of metallic silver and copper. No lines characteristic for metal oxides or copper/silver alloys were detected [14–17]. The impurities adsorbed on the substrate

3. Results and discussion The key issue in CVD of Ag/Cu layers seems to be selection of precursors, which do not react with each other in CVD process leaving pure metallic films. Cu(I) carboxylate complexes with vinyltrialkylsilanes (L) [Cu(OOCR)(L)] have been studied as copper precursors [10]. The use of Cu(I) pentafluoropropionate complex with vinyltrimethylsilane [Cu

Fig. 1. Variable temperature IR spectra of vapors formed during the thermal decomposition of 1 and 3.

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Fig. 2. SEM micrographs of Ag/Cu layers deposited using 1 and 2 as precursors; (a) surface of the film (SE detector (left), BSE one (right), (b) cross section of the film.

surface were studied by DRIFT IR. The absorption bands, 3300 and 2360 cm− 1, of water and carbon dioxide molecules, adsorbed on the substrate surface after deposition process, were observed. However the bands from fluorocarbon, hydrocarbon and carboxylate groups were not detected. The surfaces of deposited films were matt, grey with a copper color admixture. Morphology studies of Ag/Cu layers are presented in Fig. 2 (a) and (b). Analysis of micrographs exhibited the formation of the uniform, roughness layers, composed from the fine grains of silver arising film, and the large grains or rods of copper. Size of silver grains varies from 0.1 to 0.3 μm, whereas copper from 0.5

to 1.0 μm. CVD parameters analysis suggest that the length of copper rods increases with extension of deposition time. We have found that the average length of rods, deposited 60 min, were below 3000 μm, while after 120 min about 3000– 7000 μm. The differences in volatility of 1 and 3 (Table 1) were required specific deposition parameters and the process proceeds in two stages. In the first step vaporization vessels containing 1 and 3 were heated at 433 K, while substrate at 523 K, during 40 min. Variable temperature IR studies of vapors exhibited that in mentioned conditions volatile and stable Ag(I) phosphine species and only slight content of copper–carboxylate or

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Fig. 3. XRD pattern of Ag/Cu layers deposited from 1 and 3 in weight ratio (a) 1:1.5 and (b) 1.5:1.

copper–VTMS derivatives were transported. Simultaneously, the copper(I) complex (1) decomposes yielding copper(I) carboxylate. Due to, at TD = 523 K, mainly thin silver films were formed. In the next step (t = 40 min), the vaporization temperature was increased to 513 K. The transport of silver species was continued and moreover the copper carboxylate species start evaporate that was confirmed by IR studies of vapors. Simultaneously, the substrate temperature has been increased up to 713 K and finally the silver/copper materials were obtained. XRD studies confirm that prepared layers consists from metallic silver (Ag(111) 38.1°, Ag(200) 44.3°, Ag(220) 64.3° 2θ) and copper (Cu(111) 43.1°, Cu(200) 50.3°, Cu(220) 74.2° 2θ). No lines characteristic for metal oxides or copper/silver alloys were detected [14–17]. Comparison the intensity of Ag and Cu diffraction lines indicate the impact of 1:3 weight ratio (1:1.5, 1:1, and 1.5:1) (Fig. 3). Simultaneously analysis of DRIFT spectra confirms the lack of absorption bands from organic impurities (between 2700 and 3100 cm− 1). SEM microphotographs of Ag/Cu layers deposited from 1 and 3 are presented in Fig. 4 (a)–(c). The SE (Secondary Electrons) and BSE (Backscattered Electrons) detectors in SEM studies were used for copper and silver grains identification on the film surface. From the microphotograph (Fig. 4a) is evident that the Ag/ Cu film deposited from 1 and 3 in 1:1 weight ratio composes of Ag and Cu dense-packed grains with diameter 50–200 nm. The use of the excess of silver precursor (weight ratio 1:1.5) leads to the formation of copper layer composed of large grains (size 0.8–1.4 μm) covered by silver one (size 0.2–0.6 μm) (Fig. 4b). Increases of the weight ratio of silver/copper precursors to 1:2 leads to the formation of silver membrane containing copper grains (size 1.4–2.0 μm) located on the film surface (Fig. 5b). However, the material deposited using the 1.5:1 weight ratio mixture was consisted from the silver dispersed grains covered by the copper film composed of packed-density grains (Fig. 4c). The electrical conductivity, of Ag/Cu films obtained using 1 and 3 as precursors, was measured as a function of the temperature (σ(T)) by a standard four-point probe. Changes of σ(T) values of films grown on Si substrates, for which dσ/ dT N 0, in the whole measured temperature range (103–333 K), are presented in Fig. 5. As reported, the electrical resistance of

Fig. 4. SEM micrographs (SE detector (left), BSE one (right)) of Ag/Cu films fabricated from 1 and 3 using (a) 1:1, (b) 1:1.5, and (c) 1.5:1 weigh ratios (TV(1) = 513 K, TV(3) = 433 K, TD(1) = 713 K, TD(3) = 503 K, Ar, p = 150 Pa).

metal films composed from polycrystalline grains depends on point defects, impurities, grain boundaries, film surfaces and interfaces [18–21]. The weak conductivity [∼ 8.81 · 10 3 (Ω m)− 1, at 293 K], was noticed for layers composed of dense packed Ag and Cu grains deposited from 1:1 precursors

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Fig. 5. Temperature dependence of conductivity of Ag/Cu layers deposited from 1/3 and surface morphology (a) 1:1 and (b) 1:2 films (TV(1) = 513 K, TV(3) = 433 K, TD(1) = 713 K, TD(3) = 503 K, Ar, p = 150 Pa).

weight ratio (Fig. 5a). The resistances of film measured at lower temperatures increases up to ∼9.15 · 103 [(Ω m)− 1] at 100 K. The low conductivity of this film can be related to the thermal generation of carriers from the point defects. A significant conductivity increase was noted for the Ag/Cu films deposited from 1:2 weight ratio of precursors (∼ 5.99 · 104 [(Ω m)− 1] at ∼ 100 K, Fig. 5b). Analysis of the SEM image of this film exhibited the formation of the membrane, composed of linked silver grains and single metallic copper particles (Fig. 5b). The conductivity of obtained material is probably determined by the coalescence effects and formation of metallic silver membrane doped by dispersed copper grains [10,18,21]. 4. Conclusions Thin silver/copper layers were successfully prepared using carboxylate derivatives [Cu(OOCC2F5)(VTMS)] (1), [Ag (OOCC2F5)] (2), and [Ag(OOCBut)(PEt3)] (3) as precursors applied in thermal inducted CVD. The films were deposited onto silicon(111) wafers, at deposition temperatures in the range 523–713 K. Differences in the composition and thermal stability of silver and copper volatile species transported in vapors influence on the structure and morphology of Ag/Cu layers deposited from studied precursors. During the thermolysis of vapors containing Ag(I) pentafluoropropionate species, which thermal stability were similar to volatile Cu(I) carboxylate derivatives (precursors 1 and 2), the layers composed from the separate silver and copper grains. The differences in volatility of 1 and 3 and

formation of the low stable than Ag-carboxylate (2) volatile silver–phosphine species from 3 allowed the obtained different silver/copper materials depending on weight ratio of precursors 1:3. In the case of 1:1 weight ratio the layer composes of Ag and Cu dense-packed grains; for 1:1.5 — the copper layer consisted of large grains covered by silver one was formed; for 1:2 the silver membrane containing copper grains located on the film surface was obtained and when the copper precursor dominated 1.5:1 the silver dispersed grains were covered by the copper film composed of packed-density grains. Studies of electrical properties of Ag/Cu film fabricated from 1 and 3, revealed the clear influence morphology on their conductivity, measured in the temperature range 103–333 K. The highest conductivity for metallic silver membrane doped by dispersed copper grains films (1:3 = 1:2) was noticed in the range 100–130 K [∼ 5.99 · 104 (Ω m)− 1]. Acknowledgements The authors wish to acknowledge the Polish Ministry of High Education and Science for financial support grant no N204 049 31/1376. References [1] T.T. Kodas, M.J. Hampden-Smith, The Chemistry of Metal CVD, VCH, Weinheim, 1994, pp. 175–325. [2] S. Strehle, S. Menzel, H. Wendrock, J. Acker, K. Wetzig, Microelectronic Engineering 70 (2003) 506. [3] A. Benghalem, D.G. Morris, Acta Mater., 45 (1997) 397.

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[4] A. Gaganov, J. Freudenberger, E. Botcharova, L. Schultz, Materials Science and Engineering A 437 (2006) 313. [5] K. Takahashi, A. Tanaka, H. Sasaki, W. Gondo, S. Suzuki, S. Sato, Applied Surface Science 169–170 (2001) 164. [6] A. Grodzicki, I. Łakomska, P. Piszczek, I. Szymańska, E. Szłyk, Coord. Chem. Rev. 249 (2005) 2232. [7] R. Szczęsny, P. Piszczek, I. Szymańska, E. Szłyk, Annals of the Polish Chemical Society, XLVII Annual Meeting of the Polish Chemical Society, Wrocław, 2004, p. 213. [8] E. Szłyk, P. Piszczek, I. Łakomska, A. Grodzicki, J. Szatkowski, T. Błaszczyk, Chem. Vap. Dep. 6 (2000) 105. [9] T. Katagiri, E. Kondoh, N. Takeyasu, T. Nakano, Jpn. J. Appl. Phys., 32 (1993) 1078. [10] P. Piszczek, I. Szymańska, W. Bała, K. Bartkiewicz, E. Talik, J. Heiman, Thin Solid Films (2007) in press. [11] E. Szłyk, P. Piszczek, A. Grodzicki, M. Chaberski, A. Goliński, J. Szatkowski, T. Błaszczyk, Chem. Vap. Dep. 7 (2001) 111. [12] E. Szłyk, P. Piszczek, Polyhedron 20 (2001) 2853.

[13] P. Piszczek, E. Szłyk, M. Chaberski, C. Taeschner, A. Leonhard, W. Bała, K. Bartkiewicz, Chem. Vap. Dep. 11 (2005) 53. [14] Powder Diffraction File, Sets 4-836 and 5-667, Joint Committee on Diffraction Standards, 1977. [15] F. Delogu, G. Coco, Materials Science and Engineering A 402 (2005) 208–214. [16] R. Najafabadi, D.J. Srolovitz, E. Ma, M. Atzmon, J. Appl. Phys., 74 (5) (1993) 3144. [17] A. Ceylan, C. Ni, S.I. Shah, Mater. Res. Soc. Symp. Proc., 854E (2005) U5.5.1. [18] R.L. Schwöbel, J. Appl. Phys. 40 (1966) 1039. [19] H.-D. Liu, Y.-P. Zhao, G. Ramanath, S.P. Murarka, G.-C. Wang, Thin Solid Films 384 (2005) 151. [20] K.A. Yates, L.F. Cohen, C. Watine, T.-N. Tay, F. Damay, J. MacManusDriscol, R.S. Freitas, L. Ghivelder, E.M. Haines, G.A. Gehring, J. Appl. Phys. 88 (2000) 4703. [21] A.I. Gubin, A.A. Lavrinovich, N.T. Cherpak, 2001 Techn. Phys. Lett. 27 (2001) 336.