- Email: [email protected]
Tungsten chemical vapor deposition using tungsten hexacarbonyl: microstructure of as-deposited and annealed films
Thin Solid Films 370 (2000) 114±121
www.elsevier.com/locate/tsf
Tungsten chemical vapor deposition using tungsten hexacarbonyl: microstructure of as-deposited and annealed ®lms Ken K. Lai a, H. Henry Lamb a,b,* a
b
Department of Chemical Engineering, North Carolina State University, Raleigh, NC 27695-7905, USA Department of Materials Science and Engineering, North Carolina State University, Raleigh, NC 27695-7905, USA Received 26 September 1999; received in revised form 7 March 2000; accepted 7 March 2000
Abstract Tungsten (W) ®lms were deposited on Si(100) from tungsten hexacarbonyl, [W(CO)6], by low-pressure chemical vapor deposition (CVD) in an ultra-high vacuum (UHV)-compatible reactor. The chemical purity, resistivity, crystallographic phase, and morphology of the deposited ®lms depend markedly on the substrate temperature. Films deposited at 3758C contain approximately 80 at.% tungsten, 15 at.% carbon and 5 at.% oxygen. These ®lms are polycrystalline b-W with a strong (211) orientation and resistivities of .1000 mV cm. Vacuum annealing at 9008C converts the metastable b-W to polycrystalline a-W, with a resistivity of approximately 19 mV cm. The resultant a-W ®lms are porous, with small randomly oriented grains and nanoscale (,100 nm) voids. Films deposited at 5408C are highpurity (.95 at.%) polycrystalline a-W, with low resistivities (18±23 mV cm) and a tendency towards a (100) orientation. Vacuum annealing at 9008C reduces the resistivity to approximately 10 mV cm, and results in a columnar morphology with a very strong (100) orientation. q 2000 Elsevier Science S.A. All rights reserved. Keywords: Tungsten; Chemical vapor deposition; Auger electron spectroscopy; Phase transitions
1. Introduction Tungsten (W) thin ®lms are used in microelectronic and optoelectronic devices as ohmic contacts, diffusion barriers and interconnects (lines, vias and plugs) [1]. W ®lms for device applications are typically deposited by chemical vapor deposition (CVD) using WF6 and a suitable reducing agent, e.g. H2 [2]. Interfacial Si consumption and exposure of device structures to corrosive reaction byproducts, e.g. HF, are signi®cant disadvantages of conventional W CVD processes. Organometallic compounds have been investigated as W CVD precursors, but the deposited ®lms typically contain signi®cant concentrations of carbon and oxygen, resulting in unacceptably high resistivities [3]. W CVD from tungsten hexacarbonyl, [W(CO)6], has been investigated by several groups, including the original work of Kaplan and d'Heurle [4]. Recently, W ®lms deposited from [W(CO)6] have been employed successfully for the metallization of ultra-thin gate dielectrics [5]. The capability of depositing high-purity metal ®lms without damaging the underlying dielectric layer is critical to this application. * Corresponding author. Tel.: 11-919-515-6395; fax: 11-919-515-3465. E-mail address: [email protected] (H.H. Lamb).
Kaplan and d'Heurle [4] investigated W deposition from [W(CO)6] using a hot-wall, low-pressure CVD (LPCVD) reactor at 200±5808C, and hydrogen as the carrier gas. The [W(CO)6] partial pressure was varied from 5±50 mTorr. Films deposited at substrate temperatures of less than 5008C had high resistivities (200±500 mV cm) due to the presence of carbon and oxygen impurities. In contrast, W thin ®lms with resistivities of only 2±5 times the bulk value (5.6 mV cm at 208C) were deposited at temperatures of greater than or equal to 5008C. A ®lm deposited at 4508C was examined by X-ray diffraction (XRD), and found to contain polycrystalline a-W with a (100) preferred orientation. Diem et al. [6] employed a hot-wall CVD reactor, operating at 4208C and atmospheric pressure, to deposit W ®lms containing 10 and 10 at.% carbon and oxygen, respectively. Annealing the ®lms for 1 h at 8508C in vacuum or forming gas increased the purity to 96 at.% W, and lowered the resistivity to 6.6±12.5 mV cm. In this work, we have investigated the LPCVD of W ®lms from [W(CO)6] in a cold-wall, ultra-high vacuum (UHV)compatible stainless steel reactor. Films were deposited on Si(100) substrates at different substrate temperatures, reactor pressures, and deposition times. The chemical purity, electrical resistivity, crystallographic phase and preferred orientation, and morphology of the as-deposited ®lms
0040-6090/00/$ - see front matter q 2000 Elsevier Science S.A. All rights reserved. PII: S00 40-6090(00)0094 3-3
K.K. Lai, H.H. Lamb / Thin Solid Films 370 (2000) 114±121
were determined using Auger electron spectroscopy (AES) with sputter depth pro®ling, four-point probe measurements, XRD, and scanning electron microscopy (SEM), respectively. These measurements were also performed on selected ®lms after annealing in vacuum at 800±9008C. 2. Experimental The LPCVD reactor was constructed from a stainless steel six-way cross, with 4.5-inch CF ¯anges and 2.5-inch tubulation. The substrate heater consisted of a ceramic disk with a circular groove that held a tungsten heating coil. The source compounds were contained in a Kovar w-sealed Pyrex w tube that was isolated from the deposition chamber by two stainless steel bellows valves. The source compound tube was heated using heating tape. The substrate temperature was measured using a type K thermocouple that was in good contact with the back of the substrate. An Omega CN2041 programmable temperature controller and Eurotherm 831 solid state relay were used to control the substrate temperature. A turbomolecular pump (Edwards, 310 l/s), backed by a mechanical pump with a foreline liquid-N2 trap, was used for process pumping; the base pressure was 3 £ 10 27 Torr without baking. During deposition, the gate valve to the turbomolecular pump was closed, and the reaction chamber was pumped through a by-pass line containing a butter¯y valve. This allowed the reactor pressure to be controlled by adjusting the conductance of the vacuum line. The deposition pressure was measured by an MKS 127A capacitance manometer. One-inch Si(100) wafers (p-type, 8±12 V cm) were used as substrates. The wafers were cleaned using the following procedure: a 10-min UV/air treatment to remove hydrocarbons and grow a thin photochemical oxide layer, a 60-s dip in 1% HF to strip the oxide, and a 5-min DI water rinse. The cleaning procedure was repeated, and then the substrate was mounted on a sample holder, and transferred into the deposition chamber via a load lock. The load lock was evacuated using an Alcatel Drytel 31 pump, and pumped to approximately (10 27 Torr) by a Varian HV-4 cryopump. [W(CO)6) (99%) was obtained from Strem Chemicals and used without further puri®cation. Approximately 10 g of [W(CO)6] were loaded into the source tube and degassed under high vacuum at 258C. The compound was sublimed directly into the reactor without employing a carrier gas. In the temperature range of 28±358C, the [W(CO)6] vapor pressure follows the relation, log10P 12.094 2 4077/T (T in K, P in Torr), and between 80 and 1508C, it follows the relation, log10P 11.523 2 3872/T [7]. Typically, the source tube was maintained at 65±708C, which resulted in a pressure of 10±15 mTorr in the LPCVD chamber with the butter¯y valve fully open. Highly re¯ective metallic ®lms were deposited over the temperature range of 350±6008C, with typical ®lm growth periods of 1±20 min. After ®lm deposition, the samples were transferred under
115
vacuum to a surface analysis chamber for characterization by on-line AES. The UHV chamber (base pressure, 1 £ 10 210 Torr) is pumped by a Perkin±Elmer 220 l/s ion pump, and annealing of selected as-deposited ®lms was also performed in this chamber. The PHI AES 3017 system comprises a 10±155 single-pass cylindrical mirror analyzer with a coaxial electron gun and associated electronics. AES data were acquired in N(E) mode using a PHI 137 PC interface card and a 386 IBM-compatible PC. Spectra were recorded with a primary electron beam voltage of 3 kV. The atomic compositions of the ®lms were evaluated from the peak-to-peak heights of the O(KLL, 503 eV), Si(LMM, 92 eV), C(KLL, 272 eV) and W(NNN, 179 eV) AE peaks in the derivative spectrum using appropriate sensitivity factors [8]. To study the ®lm composition, AES depth pro®ling was performed with a Perkin±Elmer 04-303 differential ion gun, using a 5 kV acceleration voltage, 2.5 mA emission current, and 10 26 Torr Ar in the analysis chamber. Film thickness and morphology were examined using a JEOL 6400 ®eld emission SEM. A Rigaku Geiger¯ex X-ray diffractometer was used to determine the crystallographic phase and preferred orientation (if any) of the ®lms. Sheet resistances of the ®lms were measured using a Magne-Tron M-700 four-point probe.
3. Results A minimum substrate temperature of 3508C was required for the deposition of W ®lms on Si(100) using [W(CO)6]. The AE spectra of thin ®lms deposited at 3758C and 20 mTorr using different deposition times are shown in Fig. 1. The Si(LVV) peak arising from the substrate is attenuated gradually with increasing deposition time. During the ®rst 60 s, the surface primarily comprises silicon, carbon and oxygen. After 90 s, the W(NNN) peak has increased signi®cantly, and the C(KLL) lineshape has changed from one characteristic of graphitic/amorphous carbon to one indica-
Fig. 1. AE spectra of Si(100) after exposure to [W(CO)6] at 3758C and 20 mTorr for: (a), 0; (b), 30; (c), 60; (d), 90; (e), 120; and (f), 240 s.
116
K.K. Lai, H.H. Lamb / Thin Solid Films 370 (2000) 114±121
Table 1 Compositions of ®lms deposited from [W(CO)6] Deposition temperature/ pressure/time (8C/mTorr/min)
Annealing temperature/ time (8C/min)
Analysis region
AES elemental composition
Surface (S); ®lm (F)
W (at.%)
C (at.%)
O (at.%)
Si (at.%)
375/18/12
±
375/13/8
800/20
375/13/8
900/30
540/15/10
±
540/18/6
900/30
S F S F S F S F S F
52 78 82 81 57 92 70 95 76 96
31 15 5 13 6 4 20 4 23 3
17 7 13 6 1 2 10 1 1 1
0 0 0 0 37 2 0 0 0 0
tive of a substantial carbidic carbon component. The W surface concentration increases at the expense of Si to approximately 45 at.% after 240 s. We infer that the initial 60±90 s of deposition time represents an induction period for W deposition. The AE spectra of continuous ®lms deposited in 7 min at 3758C evidence a surface composition of approximately 55 at.% W, 30 at.% C and 15 at.% O for deposition pressures in the 8.5±20 mTorr range. At lower deposition pressures (e.g. 5 mTorr), a 7-min exposure to [W(CO)6] was not suf®cient for the nucleation and growth of continuous ®lms. The composition of a ®lm deposited at 3758C and 18 mTorr is given in Table 1. The AES depth pro®le (Fig. 2a) indicates that the ®lm is covered by a relatively thick carbon-rich layer, and suggests that there is signi®cant intermixing at the W/Si interface. These compositional gradients, however, can be affected by analytical artifacts, such as Ar 1-induced mixing and/or surface roughness effects. The SEM image (Fig. 3) of a ®lm deposited under equivalent conditions evidences a columnar growth morphology with a lateral grain dimension of 200±300 nm. The ®lm exhibits surface faceting, consistent with a highly oriented polycrystalline material. The XRD patterns of ®lms deposited at 3758C are indicative of highly oriented b-W, a low-temperature, metastable phase associated with the presence of oxygen or other main group heteroatoms [9,10]. The intense peak at 43.58 2Q in Fig. 4a is indexed to the b-W(211) plane; the Ê , which is approximately 1% computed d spacing is 2.08 A greater than the literature value for bulk b-W. Neither oxide nor carbide phases of tungsten were detected by XRD analysis of the as-deposited ®lms. Vacuum annealing of a b-W ®lm at 8008C for 20 min did not signi®cantly alter the ®lm composition; however, the W surface concentration increased markedly (Table 1). The XRD pattern (Fig. 4b) indicates the incipient conversion of the b-W phase to a-W. Subsequent annealing of the ®lm at 9008C for 30 min reduced the carbon and oxygen concentrations to less than 5 at.% (Table 1). The AES depth pro®le (Fig. 2b) evidences that 9008C annealing results in Si migration to the external
surface. The XRD pattern after 9008C annealing (Fig. 4c) indicates the complete conversion of b-W to a-W. The resultant a-W ®lm does not exhibit a preferred orientation. SEM (Fig. 5) illustrates that the converted a-W ®lm is porous; 50±100 nm voids are visible in both the surface and inclined-view images. After annealing, the ®lm contains
Fig. 2. AES depth pro®les: (a), ®lm deposited from [W(CO)6] at 3758C and 18 mTorr; and (b), ®lm deposited from [W(CO)6] at 3758C and 15 mTorr, and annealed at 9008C for 30 min.
K.K. Lai, H.H. Lamb / Thin Solid Films 370 (2000) 114±121
117
Fig. 3. Cross-sectional SEM image of a W ®lm deposited at 3758C and 18 mTorr.
small globular grains, 30±50 nm in size. Interestingly, a bW ®lm deposited on Si(111) and subjected to post-deposition annealing at 9008C exhibited a tendency towards the aW(110) orientation, suggesting that the substrate crystallographic orientation may exert some in¯uence on the morphology of the overlayer. The electrical resistivities of the as-deposited b-W ®lms,
Fig. 5. SEM images of a W ®lm deposited at 3758C and 15 mTorr, and annealed at 9008C for 30 min: (a), inclined view; and (b), surface.
Fig. 4. XRD 2u scans of ®lms deposited from [W(CO)6] at 3758C: (a), asdeposited; (b), annealed at 8008C for 20 min; and (c), annealed at 9008C for 30 min.
as determined by four-point probe measurements, were highly variable, but always greater than 1000 mV cm. A decrease in resistivity to 188 mV cm was noted upon vacuum annealing at 8008C. This observation suggests that resistivity measurements on the as-deposited ®lms were in¯uenced by the surface layer, since the 8008C anneal did not signi®cantly alter the ®lm composition or the majority crystallographic phase. The resistivity of the microporous a-W ®lm resulting from 9008C annealing was 19 mV cm. The AES depth pro®le of a ®lm deposited from [W(CO)6] at 5408C and 15 mTorr is shown in Fig. 6a. The results indicate a ®lm composition of at least 95% W, with less than 4 at.% carbon and the balance oxygen (Table 1). The surface is covered by a thin oxycarbide layer; the C(KLL) lineshape evidences that the carbon is carbidic. A typical surface composition is 70 at.% W, 20 at.% C, and 10 at.% O (Table 1). The XRD pattern in Fig. 7b demonstrates that the as-deposited ®lms are polycrystalline a-W, with a tendency towards a (100) orientation. The broadness of the a-W(200)
118
K.K. Lai, H.H. Lamb / Thin Solid Films 370 (2000) 114±121
tion at 12 mTorr, with the butter¯y valve 100% open. The deposition rate for this ®lm was signi®cantly higher (6.6 mm/h), indicating a higher [W(CO)6] conversion. The deposition rate and ®lm composition are equivalent to those of a ®lm deposited at 15 mTorr using a higher [W(CO)6] source temperature. Vacuum annealing of an as-deposited a-W ®lm at 9008C for 30 min did not signi®cantly alter the composition (Table 1). The AES depth pro®le (Fig. 6b) demonstrates that Si does not migrate through the ®lm during annealing, and that there is no additional interdiffusion at the W/Si interface. The XRD pattern of the annealed ®lm (Fig. 7a) is indicative of polycrystalline a-W with a very strong (100) orientation. The a-W(200) XRD peak has narrowed markedly relative to that of the as-deposited ®lm, consistent with grain growth and elimination of the non-uniform strain. The Ê , is equivalent to that of computed lattice parameter, 3.16 A bulk W. Tungsten silicide phases, e.g. WSi2, were not detected by XRD after 9008C annealing. The inclinedview SEM image of an annealed ®lm (Fig. 9) evidences the formation of uniform, highly oriented columnar grains. The lateral grain dimension is approximately 100 nm, and the grains terminate at the surface in a-W(110) facets. The ®lm resistivity after 9008C annealing is approximately 10 mV cm.
Fig. 6. AES depth pro®les: (a), ®lm deposited from [W(CO)6] at 5408C and 15 mTorr; and (b), ®lm deposited from [W(CO)6] at 5408C and 18 mTorr, and annealed at 9008C for 30 min.
diffraction peak (full width at half maximum; FWHM 1.18 2Q ) may be attributed to several causes, including small grain size and non-uniform strain. No other crystalline phases were detected by XRD. The cross-sectional SEM image (Fig. 8a) of the ®lm reveals a tendency towards columnar growth, but the grains are small and highly irregular. The surface image (Fig. 8b) shows faceted grains with an average lateral dimension of approximately 100 nm. The effects of [W(CO)6] ¯ux on deposition rate and ®lm properties were investigated by varying the source temperature. The average deposition rates, as determined by SEM, were 4.4, 4.8, 6.6 and 7.0 mm/h at deposition pressures of 11, 12, 15 and 18 mTorr, respectively. There was little attendant variation in ®lm composition, but the surface carbon concentration increased with increasing deposition pressure. The resistivities of the as-deposited ®lms varied from 18 to 23 mV cm. To evaluate the effects of reactor residence time on deposition rate and ®lm properties, a butter¯y valve was used to reduce the effective pumping speed of the turbomolecular pump. With the butter¯y valve 20% open, a ®lm was deposited at 26 mTorr, employing the same source temperature (55±578C) used for deposi-
Fig. 7. XRD 2u scans of ®lms deposited from [W(CO)6] at 5408C: (a), annealed at 9008C for 30 min; and (b), as-deposited.
K.K. Lai, H.H. Lamb / Thin Solid Films 370 (2000) 114±121
Fig. 8. SEM images of a W ®lm deposited at 5408C and 15 mTorr: (a), cross-section; and (b), surface.
119
grain size and presence of a-W strongly in¯uence the measured values [12,14]. The high (.1000 mV cm) resistivities of the as-deposited ®lms are likely to be due to amorphous tungsten oxide or oxycarbide impurities. Mass spectrometry studies of [W(CO)6] pyrolysis have indicated that the heteroatom impurities found in W ®lms deposited at less than 4008C are due to incomplete desorption of reaction byproducts (e.g. CO) [15]. Surface science investigations of [W(CO)6] decomposition on W(110) [16] and Ni(100) [17,18] have shed some light on the underlying mechanisms of heteroatom incorporation. [W(CO)6] decomposes spontaneously on clean W(110) and Ni(100) surfaces at sub-ambient temperatures. In contrast, the thermal decomposition of [W(CO)6] in the gas phase occurs at 1758C [7]. Xu and Zaera [17,18] found that [W(CO)6] decomposition on Ni(100) is complete at temperatures below 2258C. The surface decomposition products are W adatoms and molecularly adsorbed (a ) CO. CO desorption occurs around 1758C, the desorption temperature of CO on Ni(100). In this temperature range, however, CO dissociation competes with CO desorption, yielding adsorbed carbon and oxygen atoms. Moreover, CO dissociation is accompanied by W oxidation on Ni(100); X-ray photoelectron spectroscopy evidenced that deposited W was present as a mixture of oxides after annealing at 25±2258C. The recombination of carbon and oxygen adatoms to form gaseous CO occurs at much higher temperatures. During this process, the W adatoms on Ni(100) are reduced to the zero valent state. On W(110), recombinative CO desorption gives rise to the b 1 and b 2 desorption states at 825 and 5508C, respectively [16]. Adsorbed CO and [W(CO)6] yield equivalent total amounts of b-CO on W(110), but [W(CO)6] gives more CO desorption from the b 2 state. CO2 was not detected in the temperature-programmed reaction experiments, indicating that CO disproportionation is not a signi®cant reaction pathway. Thus, the carbon and
4. Discussion Previous investigations have shown that W ®lms deposited from [W(CO)6] at temperatures of less than 5008C are heavily contaminated with carbon and oxygen [4,6,11]. Typically, XRD characterization of these ®lms was not reported; however, Kaplan and d'Heurle [4] stated that ®lms deposited at 300±4508C from [W(CO)6] in H2 `were `pure' tungsten, without detectable carbide lines'. Our XRD results indicate that ®lms deposited at 3758C contain b-W, a low-temperature metastable phase, typically associated with heteroatom impurities. The b-W phase exhibits a cubic A3B (type A15) crystal structure with eight atoms/unit cell [9]. Metallic b-W ®lms have been deposited by a variety of nonequilibrium techniques, including r.f. sputtering [12], CVD [13] and plasma-enhanced CVD (PECVD) [14]. Films containing up to 15 at.% O have been reported to contain b-W. The resistivities of b-W ®lms are highly variable, ranging from 100 to 300 mV cm, as the impurity level,
Fig. 9. Inclined-view SEM image of a W ®lm deposited at 5408C and 18 mTorr, and annealed at 9008C for 30 min.
120
K.K. Lai, H.H. Lamb / Thin Solid Films 370 (2000) 114±121
oxygen impurities found in W ®lms deposited at temperatures of less than 4008C are due to CO dissociation. In contrast to metals, spontaneous [W(CO)6] decomposition is not observed on Si surfaces [19]. It has been suggested that decarbonylation on metals is enhanced by the charge transfer between the surface and adsorbed metal carbonyls [18]. Another factor is the heat of adsorption of CO on the metal surface (,30 kcal/mol) which lowers the activation energy for [W(CO)6] decomposition. The absence of spontaneous decomposition of [W(CO)6] may explain the induction period required for ®lm growth on Si(100) at 3758C. The AES data (Fig. 1) suggest that a critical concentration of metallic nuclei must be formed before steady-state ®lm growth can occur. Once these nucleation sites are formed, the growth of W-containing ®lms is facile. Vacuum annealing of a b-W ®lm at 8008C for 20 min yields a reduction in carbon and oxygen concentrations near the surface, and the incipient conversion of b-W to a-W. Further annealing at 9008C for 30 min results in a substantial reduction in the carbon and oxygen concentrations within the ®lm and complete conversion to a-W. The temperature of the b-W to a-W transformation is strongly dependent on the nature and concentration of heteroatom impurities; values in the literature range from 135 to 7508C [12,14,20]. Others have reported similar annealing results, but the microstructures of the as-deposited and annealed ®lms were not examined [6,21]. Diem et al. [6] suggested that the reduction in carbon and oxygen content after annealing was due to the evolution of CO that was physically trapped in molecular form within the ®lm. Alternatively, we infer that atomic C and O recombine at high temperatures to form gaseous CO. This hypothesis is supported by the available surface science data [16±18]. The a-W ®lms derived by annealing b-W ®lms at 9008C contain nanoscale pores. The AES depth pro®le after annealing indicates a high Si concentration on the external surface and a signi®cant concentration throughout the ®lm, but there is no XRD evidence of a crystalline tungsten silicide phase. Since Si migration does not occur during the annealing of as-deposited a-W ®lms, we infer that Si migration to the external surface occurs by surface diffusion within the porous structure. Si has a signi®cantly lower surface free energy than W, and the annealing temperature is greater than two-thirds of the Si bulk melting temperature [22]. CO dissociation is less favorable under high-temperature CVD conditions (5408C), and much lower concentrations of heteroatom impurities are incorporated in the ®lms, i.e. less b-CO is trapped. The AE spectra of the near-surface region shows the C(KLL) signature of W carbide. The resistivities of the as-deposited a-W ®lms are 3±4 times higher than that of bulk a-W (5.6 mV cm). This is probably due to the irregular grain structure and heteroatom contamination. The resistivities are, however, comparable to those of W ®lms deposited using WF6-based processes [23]. Annealing
of an as-deposited a-W ®lm at 9008C intensi®es the (100) orientation and produces a dense W ®lm with columnar grains and regular (110) facets. Moreover, there is no evidence of a reaction between the W ®lm and Si substrate during annealing at 9008C for 30 min. The annealed a-W ®lm has a resistivity of 10 mV cm, approximately two times the bulk value. 5. Conclusions The chemical purity, crystallographic phase and morphology of W ®lms deposited using [W(CO)6] depend markedly on the substrate temperature. Films deposited on Si(100) at 5408C are high-purity polycrystalline a-W, exhibiting a columnar growth morphology with a small (,100 nm) lateral grain dimension. These ®lms have low resistivities (18±23 mV cm), and may be useful in electronic device applications, thus avoiding the corrosion and Si consumption associated with WF6-based processes. The resistivities of the as-deposited ®lms are reduced to 10 mV cm by vacuum annealing at 9008C. The annealed ®lms are columnar with a strong (100) orientation, and exhibit (110) surface facets. Films deposited at 3758C are metastable b-W with a very strong (211) orientation. The transformation of b-W to a-W by vacuum annealing at 9008C produces ®lms with nanoscale pores. These novel porous a-W ®lms may ®nd applications as membranes, gas sensors and catalysts. Acknowledgements This work was supported by an NSF Presidential Young Investigator Award (CTS-8958350). References [1] S.P. Murarka, in: S.M. Sze (Ed.), VLSI Technology, 2nd ed., McGraw±Hill, New York, 1988. [2] J.K.J. Schmitz, Chemical Vapor Deposition of Tungsten and Tungsten Silicides, Noyes, Park Ridge, NJ, 1991. [3] A.A. Zinn, in: T.T. Kodas, M.J. Hampden-Smith (Eds.), The Chemistry of Metal CVD, VCH, Weinheim, 1994. [4] L.H. Kaplan, F.M. D'Heurle, J. Electrochem. Soc. 117 (1970) 693. [5] D.A. Buchanan, F.R. McFeely, J.J. Yurkas, Appl. Phys. Lett. 73 (1998) 1676. [6] M. Diem, M. Fisk, J. Goldman, Thin Solid Films 107 (1983) 39. [7] J.J. Lander, L.H. Germer, Trans. Met. Soc., Am. Inst. Mining Met. Eng. 175 (1948) 648. [8] L.E. Davis, N.C. MacDonald, P.W. Palmberg, G.E. Riach, R.E. Weber, Handbook of Auger Electron Spectroscopy, Physical Electronics, Eden Prairie, MN, 1976. [9] J. Donohue, The Structures of the Elements, Wiley, New York, 1974. [10] C.J. Smithells, Metals Reference Book, Interscience, London, 1955. [11] G.J. Vogt, J. Vac. Sci. Technol. 20 (1982) 1336. [12] P. Petroff, T.T. Sheng, A.K. Sinha, G.A. Rozgonyi, F.B. Alexander, J. Appl. Phys. 44 (1973) 2545. [13] H.H. Busta, C.H. Tang, J. Electrochem. Soc. 133 (1986) 1195. [14] C.C. Tang, D.W. Hess, Appl. Phys. Lett. 45 (1984) 633. [15] A.P. Patokin, V.V. Sagalovich, USSR J. Phys. Chem. 50 (1976) 370.
K.K. Lai, H.H. Lamb / Thin Solid Films 370 (2000) 114±121 [16] [17] [18] [19] [20]
F.A. Flitsch, J.R. Swanson, C.M. Friend, Surf. Sci. 245 (1991) 85. F. Zeara, J. Phys. Chem. 96 (1992) 4609. M. Xu, F. Zeara, J. Vac. Sci. Technol. A 14 (1996) 415. J.R. Swanson, F.A. Flitsch, C.M. Friend, Surf. Sci. 226 (1990) 147. P.M. Petroff, W.A. Reed, Thin Solid Films 21 (1974) 73.
121
[21] K.A. Gesheva, D.S. Gogova, G.D. Beshkov, V. Popov, Vacuum 51 (1998) 181. [22] C.R. Hammond, in: R.C. Weast (Ed.), Handbook of Chemistry and Physics, CRC Press, Boca Raton, FL, 1987. [23] M.L. Green, R.A. Levy, J. Electrochem. Soc. 132 (1985) 1243.
www.elsevier.com/locate/tsf
Tungsten chemical vapor deposition using tungsten hexacarbonyl: microstructure of as-deposited and annealed ®lms Ken K. Lai a, H. Henry Lamb a,b,* a
b
Department of Chemical Engineering, North Carolina State University, Raleigh, NC 27695-7905, USA Department of Materials Science and Engineering, North Carolina State University, Raleigh, NC 27695-7905, USA Received 26 September 1999; received in revised form 7 March 2000; accepted 7 March 2000
Abstract Tungsten (W) ®lms were deposited on Si(100) from tungsten hexacarbonyl, [W(CO)6], by low-pressure chemical vapor deposition (CVD) in an ultra-high vacuum (UHV)-compatible reactor. The chemical purity, resistivity, crystallographic phase, and morphology of the deposited ®lms depend markedly on the substrate temperature. Films deposited at 3758C contain approximately 80 at.% tungsten, 15 at.% carbon and 5 at.% oxygen. These ®lms are polycrystalline b-W with a strong (211) orientation and resistivities of .1000 mV cm. Vacuum annealing at 9008C converts the metastable b-W to polycrystalline a-W, with a resistivity of approximately 19 mV cm. The resultant a-W ®lms are porous, with small randomly oriented grains and nanoscale (,100 nm) voids. Films deposited at 5408C are highpurity (.95 at.%) polycrystalline a-W, with low resistivities (18±23 mV cm) and a tendency towards a (100) orientation. Vacuum annealing at 9008C reduces the resistivity to approximately 10 mV cm, and results in a columnar morphology with a very strong (100) orientation. q 2000 Elsevier Science S.A. All rights reserved. Keywords: Tungsten; Chemical vapor deposition; Auger electron spectroscopy; Phase transitions
1. Introduction Tungsten (W) thin ®lms are used in microelectronic and optoelectronic devices as ohmic contacts, diffusion barriers and interconnects (lines, vias and plugs) [1]. W ®lms for device applications are typically deposited by chemical vapor deposition (CVD) using WF6 and a suitable reducing agent, e.g. H2 [2]. Interfacial Si consumption and exposure of device structures to corrosive reaction byproducts, e.g. HF, are signi®cant disadvantages of conventional W CVD processes. Organometallic compounds have been investigated as W CVD precursors, but the deposited ®lms typically contain signi®cant concentrations of carbon and oxygen, resulting in unacceptably high resistivities [3]. W CVD from tungsten hexacarbonyl, [W(CO)6], has been investigated by several groups, including the original work of Kaplan and d'Heurle [4]. Recently, W ®lms deposited from [W(CO)6] have been employed successfully for the metallization of ultra-thin gate dielectrics [5]. The capability of depositing high-purity metal ®lms without damaging the underlying dielectric layer is critical to this application. * Corresponding author. Tel.: 11-919-515-6395; fax: 11-919-515-3465. E-mail address: [email protected] (H.H. Lamb).
Kaplan and d'Heurle [4] investigated W deposition from [W(CO)6] using a hot-wall, low-pressure CVD (LPCVD) reactor at 200±5808C, and hydrogen as the carrier gas. The [W(CO)6] partial pressure was varied from 5±50 mTorr. Films deposited at substrate temperatures of less than 5008C had high resistivities (200±500 mV cm) due to the presence of carbon and oxygen impurities. In contrast, W thin ®lms with resistivities of only 2±5 times the bulk value (5.6 mV cm at 208C) were deposited at temperatures of greater than or equal to 5008C. A ®lm deposited at 4508C was examined by X-ray diffraction (XRD), and found to contain polycrystalline a-W with a (100) preferred orientation. Diem et al. [6] employed a hot-wall CVD reactor, operating at 4208C and atmospheric pressure, to deposit W ®lms containing 10 and 10 at.% carbon and oxygen, respectively. Annealing the ®lms for 1 h at 8508C in vacuum or forming gas increased the purity to 96 at.% W, and lowered the resistivity to 6.6±12.5 mV cm. In this work, we have investigated the LPCVD of W ®lms from [W(CO)6] in a cold-wall, ultra-high vacuum (UHV)compatible stainless steel reactor. Films were deposited on Si(100) substrates at different substrate temperatures, reactor pressures, and deposition times. The chemical purity, electrical resistivity, crystallographic phase and preferred orientation, and morphology of the as-deposited ®lms
0040-6090/00/$ - see front matter q 2000 Elsevier Science S.A. All rights reserved. PII: S00 40-6090(00)0094 3-3
K.K. Lai, H.H. Lamb / Thin Solid Films 370 (2000) 114±121
were determined using Auger electron spectroscopy (AES) with sputter depth pro®ling, four-point probe measurements, XRD, and scanning electron microscopy (SEM), respectively. These measurements were also performed on selected ®lms after annealing in vacuum at 800±9008C. 2. Experimental The LPCVD reactor was constructed from a stainless steel six-way cross, with 4.5-inch CF ¯anges and 2.5-inch tubulation. The substrate heater consisted of a ceramic disk with a circular groove that held a tungsten heating coil. The source compounds were contained in a Kovar w-sealed Pyrex w tube that was isolated from the deposition chamber by two stainless steel bellows valves. The source compound tube was heated using heating tape. The substrate temperature was measured using a type K thermocouple that was in good contact with the back of the substrate. An Omega CN2041 programmable temperature controller and Eurotherm 831 solid state relay were used to control the substrate temperature. A turbomolecular pump (Edwards, 310 l/s), backed by a mechanical pump with a foreline liquid-N2 trap, was used for process pumping; the base pressure was 3 £ 10 27 Torr without baking. During deposition, the gate valve to the turbomolecular pump was closed, and the reaction chamber was pumped through a by-pass line containing a butter¯y valve. This allowed the reactor pressure to be controlled by adjusting the conductance of the vacuum line. The deposition pressure was measured by an MKS 127A capacitance manometer. One-inch Si(100) wafers (p-type, 8±12 V cm) were used as substrates. The wafers were cleaned using the following procedure: a 10-min UV/air treatment to remove hydrocarbons and grow a thin photochemical oxide layer, a 60-s dip in 1% HF to strip the oxide, and a 5-min DI water rinse. The cleaning procedure was repeated, and then the substrate was mounted on a sample holder, and transferred into the deposition chamber via a load lock. The load lock was evacuated using an Alcatel Drytel 31 pump, and pumped to approximately (10 27 Torr) by a Varian HV-4 cryopump. [W(CO)6) (99%) was obtained from Strem Chemicals and used without further puri®cation. Approximately 10 g of [W(CO)6] were loaded into the source tube and degassed under high vacuum at 258C. The compound was sublimed directly into the reactor without employing a carrier gas. In the temperature range of 28±358C, the [W(CO)6] vapor pressure follows the relation, log10P 12.094 2 4077/T (T in K, P in Torr), and between 80 and 1508C, it follows the relation, log10P 11.523 2 3872/T [7]. Typically, the source tube was maintained at 65±708C, which resulted in a pressure of 10±15 mTorr in the LPCVD chamber with the butter¯y valve fully open. Highly re¯ective metallic ®lms were deposited over the temperature range of 350±6008C, with typical ®lm growth periods of 1±20 min. After ®lm deposition, the samples were transferred under
115
vacuum to a surface analysis chamber for characterization by on-line AES. The UHV chamber (base pressure, 1 £ 10 210 Torr) is pumped by a Perkin±Elmer 220 l/s ion pump, and annealing of selected as-deposited ®lms was also performed in this chamber. The PHI AES 3017 system comprises a 10±155 single-pass cylindrical mirror analyzer with a coaxial electron gun and associated electronics. AES data were acquired in N(E) mode using a PHI 137 PC interface card and a 386 IBM-compatible PC. Spectra were recorded with a primary electron beam voltage of 3 kV. The atomic compositions of the ®lms were evaluated from the peak-to-peak heights of the O(KLL, 503 eV), Si(LMM, 92 eV), C(KLL, 272 eV) and W(NNN, 179 eV) AE peaks in the derivative spectrum using appropriate sensitivity factors [8]. To study the ®lm composition, AES depth pro®ling was performed with a Perkin±Elmer 04-303 differential ion gun, using a 5 kV acceleration voltage, 2.5 mA emission current, and 10 26 Torr Ar in the analysis chamber. Film thickness and morphology were examined using a JEOL 6400 ®eld emission SEM. A Rigaku Geiger¯ex X-ray diffractometer was used to determine the crystallographic phase and preferred orientation (if any) of the ®lms. Sheet resistances of the ®lms were measured using a Magne-Tron M-700 four-point probe.
3. Results A minimum substrate temperature of 3508C was required for the deposition of W ®lms on Si(100) using [W(CO)6]. The AE spectra of thin ®lms deposited at 3758C and 20 mTorr using different deposition times are shown in Fig. 1. The Si(LVV) peak arising from the substrate is attenuated gradually with increasing deposition time. During the ®rst 60 s, the surface primarily comprises silicon, carbon and oxygen. After 90 s, the W(NNN) peak has increased signi®cantly, and the C(KLL) lineshape has changed from one characteristic of graphitic/amorphous carbon to one indica-
Fig. 1. AE spectra of Si(100) after exposure to [W(CO)6] at 3758C and 20 mTorr for: (a), 0; (b), 30; (c), 60; (d), 90; (e), 120; and (f), 240 s.
116
K.K. Lai, H.H. Lamb / Thin Solid Films 370 (2000) 114±121
Table 1 Compositions of ®lms deposited from [W(CO)6] Deposition temperature/ pressure/time (8C/mTorr/min)
Annealing temperature/ time (8C/min)
Analysis region
AES elemental composition
Surface (S); ®lm (F)
W (at.%)
C (at.%)
O (at.%)
Si (at.%)
375/18/12
±
375/13/8
800/20
375/13/8
900/30
540/15/10
±
540/18/6
900/30
S F S F S F S F S F
52 78 82 81 57 92 70 95 76 96
31 15 5 13 6 4 20 4 23 3
17 7 13 6 1 2 10 1 1 1
0 0 0 0 37 2 0 0 0 0
tive of a substantial carbidic carbon component. The W surface concentration increases at the expense of Si to approximately 45 at.% after 240 s. We infer that the initial 60±90 s of deposition time represents an induction period for W deposition. The AE spectra of continuous ®lms deposited in 7 min at 3758C evidence a surface composition of approximately 55 at.% W, 30 at.% C and 15 at.% O for deposition pressures in the 8.5±20 mTorr range. At lower deposition pressures (e.g. 5 mTorr), a 7-min exposure to [W(CO)6] was not suf®cient for the nucleation and growth of continuous ®lms. The composition of a ®lm deposited at 3758C and 18 mTorr is given in Table 1. The AES depth pro®le (Fig. 2a) indicates that the ®lm is covered by a relatively thick carbon-rich layer, and suggests that there is signi®cant intermixing at the W/Si interface. These compositional gradients, however, can be affected by analytical artifacts, such as Ar 1-induced mixing and/or surface roughness effects. The SEM image (Fig. 3) of a ®lm deposited under equivalent conditions evidences a columnar growth morphology with a lateral grain dimension of 200±300 nm. The ®lm exhibits surface faceting, consistent with a highly oriented polycrystalline material. The XRD patterns of ®lms deposited at 3758C are indicative of highly oriented b-W, a low-temperature, metastable phase associated with the presence of oxygen or other main group heteroatoms [9,10]. The intense peak at 43.58 2Q in Fig. 4a is indexed to the b-W(211) plane; the Ê , which is approximately 1% computed d spacing is 2.08 A greater than the literature value for bulk b-W. Neither oxide nor carbide phases of tungsten were detected by XRD analysis of the as-deposited ®lms. Vacuum annealing of a b-W ®lm at 8008C for 20 min did not signi®cantly alter the ®lm composition; however, the W surface concentration increased markedly (Table 1). The XRD pattern (Fig. 4b) indicates the incipient conversion of the b-W phase to a-W. Subsequent annealing of the ®lm at 9008C for 30 min reduced the carbon and oxygen concentrations to less than 5 at.% (Table 1). The AES depth pro®le (Fig. 2b) evidences that 9008C annealing results in Si migration to the external
surface. The XRD pattern after 9008C annealing (Fig. 4c) indicates the complete conversion of b-W to a-W. The resultant a-W ®lm does not exhibit a preferred orientation. SEM (Fig. 5) illustrates that the converted a-W ®lm is porous; 50±100 nm voids are visible in both the surface and inclined-view images. After annealing, the ®lm contains
Fig. 2. AES depth pro®les: (a), ®lm deposited from [W(CO)6] at 3758C and 18 mTorr; and (b), ®lm deposited from [W(CO)6] at 3758C and 15 mTorr, and annealed at 9008C for 30 min.
K.K. Lai, H.H. Lamb / Thin Solid Films 370 (2000) 114±121
117
Fig. 3. Cross-sectional SEM image of a W ®lm deposited at 3758C and 18 mTorr.
small globular grains, 30±50 nm in size. Interestingly, a bW ®lm deposited on Si(111) and subjected to post-deposition annealing at 9008C exhibited a tendency towards the aW(110) orientation, suggesting that the substrate crystallographic orientation may exert some in¯uence on the morphology of the overlayer. The electrical resistivities of the as-deposited b-W ®lms,
Fig. 5. SEM images of a W ®lm deposited at 3758C and 15 mTorr, and annealed at 9008C for 30 min: (a), inclined view; and (b), surface.
Fig. 4. XRD 2u scans of ®lms deposited from [W(CO)6] at 3758C: (a), asdeposited; (b), annealed at 8008C for 20 min; and (c), annealed at 9008C for 30 min.
as determined by four-point probe measurements, were highly variable, but always greater than 1000 mV cm. A decrease in resistivity to 188 mV cm was noted upon vacuum annealing at 8008C. This observation suggests that resistivity measurements on the as-deposited ®lms were in¯uenced by the surface layer, since the 8008C anneal did not signi®cantly alter the ®lm composition or the majority crystallographic phase. The resistivity of the microporous a-W ®lm resulting from 9008C annealing was 19 mV cm. The AES depth pro®le of a ®lm deposited from [W(CO)6] at 5408C and 15 mTorr is shown in Fig. 6a. The results indicate a ®lm composition of at least 95% W, with less than 4 at.% carbon and the balance oxygen (Table 1). The surface is covered by a thin oxycarbide layer; the C(KLL) lineshape evidences that the carbon is carbidic. A typical surface composition is 70 at.% W, 20 at.% C, and 10 at.% O (Table 1). The XRD pattern in Fig. 7b demonstrates that the as-deposited ®lms are polycrystalline a-W, with a tendency towards a (100) orientation. The broadness of the a-W(200)
118
K.K. Lai, H.H. Lamb / Thin Solid Films 370 (2000) 114±121
tion at 12 mTorr, with the butter¯y valve 100% open. The deposition rate for this ®lm was signi®cantly higher (6.6 mm/h), indicating a higher [W(CO)6] conversion. The deposition rate and ®lm composition are equivalent to those of a ®lm deposited at 15 mTorr using a higher [W(CO)6] source temperature. Vacuum annealing of an as-deposited a-W ®lm at 9008C for 30 min did not signi®cantly alter the composition (Table 1). The AES depth pro®le (Fig. 6b) demonstrates that Si does not migrate through the ®lm during annealing, and that there is no additional interdiffusion at the W/Si interface. The XRD pattern of the annealed ®lm (Fig. 7a) is indicative of polycrystalline a-W with a very strong (100) orientation. The a-W(200) XRD peak has narrowed markedly relative to that of the as-deposited ®lm, consistent with grain growth and elimination of the non-uniform strain. The Ê , is equivalent to that of computed lattice parameter, 3.16 A bulk W. Tungsten silicide phases, e.g. WSi2, were not detected by XRD after 9008C annealing. The inclinedview SEM image of an annealed ®lm (Fig. 9) evidences the formation of uniform, highly oriented columnar grains. The lateral grain dimension is approximately 100 nm, and the grains terminate at the surface in a-W(110) facets. The ®lm resistivity after 9008C annealing is approximately 10 mV cm.
Fig. 6. AES depth pro®les: (a), ®lm deposited from [W(CO)6] at 5408C and 15 mTorr; and (b), ®lm deposited from [W(CO)6] at 5408C and 18 mTorr, and annealed at 9008C for 30 min.
diffraction peak (full width at half maximum; FWHM 1.18 2Q ) may be attributed to several causes, including small grain size and non-uniform strain. No other crystalline phases were detected by XRD. The cross-sectional SEM image (Fig. 8a) of the ®lm reveals a tendency towards columnar growth, but the grains are small and highly irregular. The surface image (Fig. 8b) shows faceted grains with an average lateral dimension of approximately 100 nm. The effects of [W(CO)6] ¯ux on deposition rate and ®lm properties were investigated by varying the source temperature. The average deposition rates, as determined by SEM, were 4.4, 4.8, 6.6 and 7.0 mm/h at deposition pressures of 11, 12, 15 and 18 mTorr, respectively. There was little attendant variation in ®lm composition, but the surface carbon concentration increased with increasing deposition pressure. The resistivities of the as-deposited ®lms varied from 18 to 23 mV cm. To evaluate the effects of reactor residence time on deposition rate and ®lm properties, a butter¯y valve was used to reduce the effective pumping speed of the turbomolecular pump. With the butter¯y valve 20% open, a ®lm was deposited at 26 mTorr, employing the same source temperature (55±578C) used for deposi-
Fig. 7. XRD 2u scans of ®lms deposited from [W(CO)6] at 5408C: (a), annealed at 9008C for 30 min; and (b), as-deposited.
K.K. Lai, H.H. Lamb / Thin Solid Films 370 (2000) 114±121
Fig. 8. SEM images of a W ®lm deposited at 5408C and 15 mTorr: (a), cross-section; and (b), surface.
119
grain size and presence of a-W strongly in¯uence the measured values [12,14]. The high (.1000 mV cm) resistivities of the as-deposited ®lms are likely to be due to amorphous tungsten oxide or oxycarbide impurities. Mass spectrometry studies of [W(CO)6] pyrolysis have indicated that the heteroatom impurities found in W ®lms deposited at less than 4008C are due to incomplete desorption of reaction byproducts (e.g. CO) [15]. Surface science investigations of [W(CO)6] decomposition on W(110) [16] and Ni(100) [17,18] have shed some light on the underlying mechanisms of heteroatom incorporation. [W(CO)6] decomposes spontaneously on clean W(110) and Ni(100) surfaces at sub-ambient temperatures. In contrast, the thermal decomposition of [W(CO)6] in the gas phase occurs at 1758C [7]. Xu and Zaera [17,18] found that [W(CO)6] decomposition on Ni(100) is complete at temperatures below 2258C. The surface decomposition products are W adatoms and molecularly adsorbed (a ) CO. CO desorption occurs around 1758C, the desorption temperature of CO on Ni(100). In this temperature range, however, CO dissociation competes with CO desorption, yielding adsorbed carbon and oxygen atoms. Moreover, CO dissociation is accompanied by W oxidation on Ni(100); X-ray photoelectron spectroscopy evidenced that deposited W was present as a mixture of oxides after annealing at 25±2258C. The recombination of carbon and oxygen adatoms to form gaseous CO occurs at much higher temperatures. During this process, the W adatoms on Ni(100) are reduced to the zero valent state. On W(110), recombinative CO desorption gives rise to the b 1 and b 2 desorption states at 825 and 5508C, respectively [16]. Adsorbed CO and [W(CO)6] yield equivalent total amounts of b-CO on W(110), but [W(CO)6] gives more CO desorption from the b 2 state. CO2 was not detected in the temperature-programmed reaction experiments, indicating that CO disproportionation is not a signi®cant reaction pathway. Thus, the carbon and
4. Discussion Previous investigations have shown that W ®lms deposited from [W(CO)6] at temperatures of less than 5008C are heavily contaminated with carbon and oxygen [4,6,11]. Typically, XRD characterization of these ®lms was not reported; however, Kaplan and d'Heurle [4] stated that ®lms deposited at 300±4508C from [W(CO)6] in H2 `were `pure' tungsten, without detectable carbide lines'. Our XRD results indicate that ®lms deposited at 3758C contain b-W, a low-temperature metastable phase, typically associated with heteroatom impurities. The b-W phase exhibits a cubic A3B (type A15) crystal structure with eight atoms/unit cell [9]. Metallic b-W ®lms have been deposited by a variety of nonequilibrium techniques, including r.f. sputtering [12], CVD [13] and plasma-enhanced CVD (PECVD) [14]. Films containing up to 15 at.% O have been reported to contain b-W. The resistivities of b-W ®lms are highly variable, ranging from 100 to 300 mV cm, as the impurity level,
Fig. 9. Inclined-view SEM image of a W ®lm deposited at 5408C and 18 mTorr, and annealed at 9008C for 30 min.
120
K.K. Lai, H.H. Lamb / Thin Solid Films 370 (2000) 114±121
oxygen impurities found in W ®lms deposited at temperatures of less than 4008C are due to CO dissociation. In contrast to metals, spontaneous [W(CO)6] decomposition is not observed on Si surfaces [19]. It has been suggested that decarbonylation on metals is enhanced by the charge transfer between the surface and adsorbed metal carbonyls [18]. Another factor is the heat of adsorption of CO on the metal surface (,30 kcal/mol) which lowers the activation energy for [W(CO)6] decomposition. The absence of spontaneous decomposition of [W(CO)6] may explain the induction period required for ®lm growth on Si(100) at 3758C. The AES data (Fig. 1) suggest that a critical concentration of metallic nuclei must be formed before steady-state ®lm growth can occur. Once these nucleation sites are formed, the growth of W-containing ®lms is facile. Vacuum annealing of a b-W ®lm at 8008C for 20 min yields a reduction in carbon and oxygen concentrations near the surface, and the incipient conversion of b-W to a-W. Further annealing at 9008C for 30 min results in a substantial reduction in the carbon and oxygen concentrations within the ®lm and complete conversion to a-W. The temperature of the b-W to a-W transformation is strongly dependent on the nature and concentration of heteroatom impurities; values in the literature range from 135 to 7508C [12,14,20]. Others have reported similar annealing results, but the microstructures of the as-deposited and annealed ®lms were not examined [6,21]. Diem et al. [6] suggested that the reduction in carbon and oxygen content after annealing was due to the evolution of CO that was physically trapped in molecular form within the ®lm. Alternatively, we infer that atomic C and O recombine at high temperatures to form gaseous CO. This hypothesis is supported by the available surface science data [16±18]. The a-W ®lms derived by annealing b-W ®lms at 9008C contain nanoscale pores. The AES depth pro®le after annealing indicates a high Si concentration on the external surface and a signi®cant concentration throughout the ®lm, but there is no XRD evidence of a crystalline tungsten silicide phase. Since Si migration does not occur during the annealing of as-deposited a-W ®lms, we infer that Si migration to the external surface occurs by surface diffusion within the porous structure. Si has a signi®cantly lower surface free energy than W, and the annealing temperature is greater than two-thirds of the Si bulk melting temperature [22]. CO dissociation is less favorable under high-temperature CVD conditions (5408C), and much lower concentrations of heteroatom impurities are incorporated in the ®lms, i.e. less b-CO is trapped. The AE spectra of the near-surface region shows the C(KLL) signature of W carbide. The resistivities of the as-deposited a-W ®lms are 3±4 times higher than that of bulk a-W (5.6 mV cm). This is probably due to the irregular grain structure and heteroatom contamination. The resistivities are, however, comparable to those of W ®lms deposited using WF6-based processes [23]. Annealing
of an as-deposited a-W ®lm at 9008C intensi®es the (100) orientation and produces a dense W ®lm with columnar grains and regular (110) facets. Moreover, there is no evidence of a reaction between the W ®lm and Si substrate during annealing at 9008C for 30 min. The annealed a-W ®lm has a resistivity of 10 mV cm, approximately two times the bulk value. 5. Conclusions The chemical purity, crystallographic phase and morphology of W ®lms deposited using [W(CO)6] depend markedly on the substrate temperature. Films deposited on Si(100) at 5408C are high-purity polycrystalline a-W, exhibiting a columnar growth morphology with a small (,100 nm) lateral grain dimension. These ®lms have low resistivities (18±23 mV cm), and may be useful in electronic device applications, thus avoiding the corrosion and Si consumption associated with WF6-based processes. The resistivities of the as-deposited ®lms are reduced to 10 mV cm by vacuum annealing at 9008C. The annealed ®lms are columnar with a strong (100) orientation, and exhibit (110) surface facets. Films deposited at 3758C are metastable b-W with a very strong (211) orientation. The transformation of b-W to a-W by vacuum annealing at 9008C produces ®lms with nanoscale pores. These novel porous a-W ®lms may ®nd applications as membranes, gas sensors and catalysts. Acknowledgements This work was supported by an NSF Presidential Young Investigator Award (CTS-8958350). References [1] S.P. Murarka, in: S.M. Sze (Ed.), VLSI Technology, 2nd ed., McGraw±Hill, New York, 1988. [2] J.K.J. Schmitz, Chemical Vapor Deposition of Tungsten and Tungsten Silicides, Noyes, Park Ridge, NJ, 1991. [3] A.A. Zinn, in: T.T. Kodas, M.J. Hampden-Smith (Eds.), The Chemistry of Metal CVD, VCH, Weinheim, 1994. [4] L.H. Kaplan, F.M. D'Heurle, J. Electrochem. Soc. 117 (1970) 693. [5] D.A. Buchanan, F.R. McFeely, J.J. Yurkas, Appl. Phys. Lett. 73 (1998) 1676. [6] M. Diem, M. Fisk, J. Goldman, Thin Solid Films 107 (1983) 39. [7] J.J. Lander, L.H. Germer, Trans. Met. Soc., Am. Inst. Mining Met. Eng. 175 (1948) 648. [8] L.E. Davis, N.C. MacDonald, P.W. Palmberg, G.E. Riach, R.E. Weber, Handbook of Auger Electron Spectroscopy, Physical Electronics, Eden Prairie, MN, 1976. [9] J. Donohue, The Structures of the Elements, Wiley, New York, 1974. [10] C.J. Smithells, Metals Reference Book, Interscience, London, 1955. [11] G.J. Vogt, J. Vac. Sci. Technol. 20 (1982) 1336. [12] P. Petroff, T.T. Sheng, A.K. Sinha, G.A. Rozgonyi, F.B. Alexander, J. Appl. Phys. 44 (1973) 2545. [13] H.H. Busta, C.H. Tang, J. Electrochem. Soc. 133 (1986) 1195. [14] C.C. Tang, D.W. Hess, Appl. Phys. Lett. 45 (1984) 633. [15] A.P. Patokin, V.V. Sagalovich, USSR J. Phys. Chem. 50 (1976) 370.
K.K. Lai, H.H. Lamb / Thin Solid Films 370 (2000) 114±121 [16] [17] [18] [19] [20]
F.A. Flitsch, J.R. Swanson, C.M. Friend, Surf. Sci. 245 (1991) 85. F. Zeara, J. Phys. Chem. 96 (1992) 4609. M. Xu, F. Zeara, J. Vac. Sci. Technol. A 14 (1996) 415. J.R. Swanson, F.A. Flitsch, C.M. Friend, Surf. Sci. 226 (1990) 147. P.M. Petroff, W.A. Reed, Thin Solid Films 21 (1974) 73.
121
[21] K.A. Gesheva, D.S. Gogova, G.D. Beshkov, V. Popov, Vacuum 51 (1998) 181. [22] C.R. Hammond, in: R.C. Weast (Ed.), Handbook of Chemistry and Physics, CRC Press, Boca Raton, FL, 1987. [23] M.L. Green, R.A. Levy, J. Electrochem. Soc. 132 (1985) 1243.