W(IV) Cp HYDRIDES | mocvd-precursor-encyclopedia.de

TUNGSTEN (IV) CYCLOPENTADIENYLS HYDRIDES

Tungsten bis(cyclopentadienyl) bis(hydride) WH2Cp2

Fig. Molecular structure of WH2Cp2

Tungsten bis(cyclopentadienyl) bis(hydride) WH2Cp2 (M=316.04) is very reactive orange colored solid (eliminating H2 on heating), melting point 115°C (other data 189-190°C), vapor pressure 1 Torr/ 146°C, decomposition temperature onset (CVD) 280-300°C.

WH2Cp2 is synthesized by the reduction of tungsten hexachloride or tungsten pentachloride by sodium borohydride or lithium alumohydride in the presence of sodium cyclopentadienyl in THF (yield: 65 – 75 %):

WCl6 + 4 NaCp + 2 Na[BH4] → WH2Cp2 + (C5H5)2 + B2H6 + 6 NaCl

WCl5 + 2NaCp + 3H- → WH2Cp2 + 2NaCl + 3Cl- + ½ H2 (H- source: Li[AlH4], Na[BH4])

WH2(η5-C5H5)2 for WO3 films by MOCVD

WH2Cp2 was applied as precursor for the growth of WO3 films on Si(100), SiO2, Si/Pt(1–3 μm) substrates by MOCVD in the hot-wall CVD at 300 – 650 °C growth temperatures, atmospheric pressure, H2 or Ar carrier gas. WH2(η5-C5H5)2 precursor was evaporated at 80 – 130 °C and was transported to the growth zone by 20 – 25 sccm H2 carrier gas. (feed gas lines kept at 80 – 130 °C). At 360 – 380 °C growth temperatures bright, mirrorlike 145 nm thick W layer was obtained after 8 hours deposition time (growth rates 0.3 – 0.9 nm/min), Scotch-Tape test revealed excellent adhesion. Amorphous W films could be converted to crystalline W layers at 600 – 650 °C temperatures. O and C contamination was found in the deposited layers (each ca. 5 at. %): oxygen comes probably from the etching of SiO2 surface by H2). Resistivity of deposited W films was 150 – 250 μWcm (higher value was explained by high C content and amorphous nature of grown layers). Below 250 °C no deposition was observed; at deposition temperatures 330 – 360 °C some reactions with H2 evolution occurred, but most of the precursor passed heating zone unchanged (indicating relative stability of the WH2Cp2 precursor). Above 380 °C: the higher temperature, the higher contamination with C and O was observed. Deposition of W occurred more easily on Si compared to SiO2 (glass).

WH2Cp2 for W/Pt films by MOCVD

High purity W films were obtained by MOCVD from WH2Cp2 precursor when PtMe3Cp catalyst was added to the film, carried out in the horizontal reactor (2.5 cm glass tube). W coatings were deposited on glass and Si(100) substrates at 380°±20° C. WH2Cp2 precursor was evaporated at 100°-150° C (at these temperatures vapor pressure of WH2Cp2 is ~0.01 Torr); carrier gas consisting of 8 sccm H2 and 16sccm Ar was used. (Ar was passed through a glass frit containing PtMe3Cp precursor at 23° C and was saturated with 0.045 Torr of the PtMe3Cp precursor, resulting in relative ratios Cp2WH2/ CpPtMe3 = 90/10 mol %); growth time was 6-20 hours. The growth rate of the films was 0.05-0.15 Å/s. WH2Cp2 decomposed more readily on Si(100) compared to the glass substrates (for example, after 6 hours of deposition the glass surface was covered with a transparent brown layer , whereas the silicon was covered with a uniform, highly reflective metal W film); the scotch tape test demonstrated excellent adherence of grown W films to the Si substrate.

The deposited W/Pt films were analysed by XRD, SEM, AES with depth profiling (structure and composition); sheet resistivity was determined by 4-point probe, layer thickness was determined by depth profiling measurement. The grown layers were amorphous and consisted of 10-50 nm clusters by SEM.; annealing in hydrogen at 750° C allowed to convert the clusters into microcrystallites. The grown W/Pt film was analysed by AES; it was found to contain 89.6% W, 3.3% Pt, 5.3% C, and 1.8% O. For comparison, the composition of W layers deposited without the organoplatinum catalyst was 71.8% W, 25.1% C, and 3.1% O). Thus, significantly decreased carbon and oxygen contamination was obtained when the precursor catalytic metal compound is codeposited with the tungsten precursor.(relatively pure tungsten film produced by thermal decomposition of an organotungsten precursor – explained to be a consequence of the catalytic hydrogenation and hydrogenolysis of the hydrocarbon ligands by the codeposited platinum).

The XRD pattern of W film deposited on Si(100) after annealing in H2 at 750° C. for 2.5 hours. contained sharp lines near 2θ=40°, 58°, 73°, and 87° (due to W metal) (Si. A substrate showed a broad peak at 69°); no peaks of tungsten silicide WSix were found.

Unannealed W films had sheet resistivity ~54±4 μΩ.cm (significantly higher compared to the 5.6 μΩ.cm value for the bulk W metal. (high resistivity is probably due to the amorphous nature of the deposit and poor contact between adjacent metal clusters). [i]

[i] H.D. Kaesz, R.F. Hicks, US 5403620 A, 1995, “Catalysis in organometallic CVD of thin metal films”, http://www.google.com/patents/US5403620

WH2Cp2 for W/Pt films by laser induced CVD

WH2Cp2 (combined with PtMe3Cp catalyst) was applied as well for the laser photodeposition (laser enhanced MOCVD) of W/Pt thin films on glass, silicon, fused silica, sapphire (001), GaAs (100) substrates (The 308 nm line of a pulsed XeCl excimer laser (2.6 mJ/pulse at 10 Hz) or the 351 and 364 nm lines of CW Ar ion laser (4-5 mW/mm2) were used for laser excitation of precursor). Ar carrier gas was separately saturated over crystals of WH2Cp2 and over crystals of PtMe3Cp, then H2 was introduced into the reaction chamber in proximity to a laser beam; the photolysis of WH2Cp2 precursor was carried out at atmospheric pressure with the laser beam perpendicular to the surface (an alternative deposition using the laser beam parallel to the surface or at intermediate angles to induce a gas phase reaction could also be used). Ca. 100 nm W layers were formed after 10 minutes (growth rate 0.6µ/h), the layers had similar good purity by AES.[i]

[i] H.D. Kaesz, R.F. Hicks, US 5403620 A, 1995, “Catalysis in organometallic CVD of thin metal films”, http://www.google.com/patents/US5403620

Bis(methylcyclopentadienyl) tungsten dihydride WH2(η5-MeCp)2

Bis(methylcyclopentadienyl) tungsten dihydride WH2(MeCp)2 is solid with melting point 93-94°C, vapor pressure 1 Torr/ 117°C, decomposition temperature onset (in CVD conditions) ~200°C .

Bis(methylcyclopentadienyl) tungsten dihydride is synthesized by the reduction of tungsten hexachloride or pentachloride by sodium borohydride or lithium alumohydride in the presence of sodium methylcyclopentadienyl in THF (yield: 65 – 75 %)

WCl6 + 4 Na(MeCp) + 2 Na[BH4] → WH2(MeCp)2 + (C5H4CH3)2 +B2H6 + 6 NaCl

WCl5 + 2 Na(MeCp) + 3 H- → WH2(MeCp)2 + 2 NaCl + 3 Cl- + ½ H2

(H- source: Li[AlH4], Na[BH4])

Bis(methylcyclopentadienyl) tungsten dihydride has been applied as MOCVD precursor, the deposition conditions are same as described for WH2Cp2. Advantage of WH2(MeCp)2 over WH2Cp2 is much lower melting point (94 – 95 °C) and higher stability against oxidation (easier storage and handling). WH2(MeCp)2 can be used kept in the liquid state at ca. 100°C in a bubbler. Properties of the tungsten films obtained from WH2(MeCp)2 precursor are similar to those obtained with WH2Cp2, no extra carbon contamination was observed in the film due to the CH3 group present. Addition of methyl group does not result in lower deposition temperatures and faster film growth.

Tungsten bis(ethylcyclopentadienyl) bis(hydride) WH2(EtCp)2

Tungsten bis(ethylcyclopentadienyl) bis(hydride) WH2(EtCp)2 has been analysed by vapor pressure measurements and thermogravimetry/DSC/DTA.

WH2(EtCp)2 (and for comparson WH2(iPrCp)2) has been tested as novel precursor for MOCVD and ALD of WCx and WNxCy films. However, oxygen containing, porous and amorphous WCx (x ~ 0.5) and WNxCy (x ~0.4, y ~0.2) were obtained. [i]

[i] A. C. Anacleto, N. Blasco, A. Pinchart, Y. Marot, Ch. Lachaud, Surf. Coat. Tec., 2007, vol. 201, iss. 22-23, 25 September 2007, p. 9120-9124, Euro CVD 16, 16th European Conference on Chemical Vapor Deposition

Tungsten bis(isopropylcyclopentadienyl) bis(hydride) WH2(iPrCp)2

Tungsten bis(isopropylcyclopentadienyl) bis(hydride) WH2(iPrCp)2 (M = 400.20) has boiling point 230°C, n20/D = 1.6, it was studied by thermogravimetry/DSC/DTA and vapor pressure measurements.

WH2(iPrCp)2 (and for comparison WH2(EtCp)2) was tested as novel precursor for MOCVD and ALD of WCx and WNxCy films. Kinetics of the surface reaction using WH2(iPrCp)2 precursor has been evaluated carrying out MOCVD at low temperature ranging 350°C to 400°C; on-line QMS analysis of the deposition process was used to characterize the precursor decomposition pathway. Oxygen containing, porous and amorphous WCx (x ~ 0.5) and WNxCy (x ~0.4, y ~0.2) were obtained.[i]

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