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Surface characterization of CVD tungsten coating on molybdenum substrate
Surface & Coatings Technology 198 (2005) 169 – 172 www.elsevier.com/locate/surfcoat
Surface characterization of CVD tungsten coating on molybdenum substrate Du Jihong*, Li Zhengxiang, Liu Gaojian, Zhou Hui, Huang Chunliang Northwest Institute for Nonferrous Metal Research, Weiyang Road 96, Xi’an, Shannxi 710016, PR China Available online 1 January 2005
Abstract Surface characterization and microstructure studies are performed on chemical vapor deposited (CVD) tungsten coating. There is about 2-Am-thick diffusion layer of tungsten in the molybdenum substrate. The thermal shock test shows that tungsten coating has good adhesion with molybdenum substrate, but the elements of oxygen and carbon in the tungsten coating have the bad effects to the adhesion. The result of high-temperature diffusion experiment in the diffusion rate from molybdenum substrate to tungsten coating is faster. D 2004 Published by Elsevier B.V. Keywords: Chemical vapor deposited; Molybdenum substrate; Tungsten coating
1. Introduction Tungsten has good thermal conductivity and the highest melting point of all metals. These physical properties make it widely use in the electronic industry and spaceflight application. Chemical vapor deposition (CVD) is a coating method that utilizes the decomposition of a gaseous precursor, flowed over or through a heated substrate, with subsequent condensation from the vapor state to solid deposit. Tungsten film can be prepared in several ways. Evaporation, sputtering, and CVD are the most commonly used processes in industry. CVD has several advantages over evaporation and sputtering, including better step coverage and low coating time. The equipment necessary to obtain high-quality films is also relatively economical in comparison to other techniques. Additionally, tungsten can be deposited selectively on desired areas under certain conditions to remove postprocessing steps such as masking and
* Corresponding author. E-mail address: [email protected] (D. Jihong). 0257-8972/$ - see front matter D 2004 Published by Elsevier B.V. doi:10.1016/j.surfcoat.2004.10.130
etching. Tungsten deposition is generally accomplished industrially by the reduction of tungsten hexafluoride or chloride tungsten by hydrogen gas [1–3]. In the present work, the properties and the adhesion of CVD tungsten coating were studied. The interface of tungsten coated molybdenum was observed by scanning electron microscopy. Thermal shock tests were performed in order to elucidate the durability of the coated layer.
2. Experiment Schematic diagram of CVD apparatus is shown in Fig. 1. The tungsten chloride used in our experiments was produced by the chlorination of tungsten in a quartz tube which was heated in a furnace to 850–950 8C. The flow rate of gases being measured by a mass flow meter. The reaction gases were pumped away using a rotatory pump. The substrate was rotated during the deposition process to yield a deposit of uniform thickness. The samples were molybdenum tube (U20600). The surface structure elements and corresponding crystal orientations of the deposited layer was investigated
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3. Results and discussion 3.1. The cross-sectional microstructure and the crystal orientation of deposited tungsten layers
Fig. 1. Schematic diagram of CVD apparatus. 1-hydrogen entrance; 2chlorine entrance; 3-tungsten powders; 4-specimen; 5-induction coil; 6rotary pole; 7-waste gases exit; 8-rotary system.
using scanning electron microscope. The thermal shock properties of deposited layers are studied after 25–1400 8C cycling of 20 times. The high-temperature diffusibility of deposited layers was studied by heating at 1800 8C for 2 h.
Fig. 2 shows the electron microprobe scanning curve in the tungsten/molybdenum diffusion layer. There is a mutual diffused layer between tungsten coatings and the molybdenum substrate; the thickness of the diffusion layer is about 2 Am. The diffusion depth of molybdenum into the tungsten layer is deeper. Since molybdenum and tungsten have the same crystal structure (b.c.c.), the similar radius, and the total mutual solvability, they could mutually diffuse in hightemperature annealing. The self-diffusion activation energy of metal has constant ratio to melting point temperature and the melting point temperature of tungsten is higher than that of molybdenum, so that the self-diffusion activation energy of tungsten is higher, and the diffusion rate of molybdenum in tungsten layer is faster [4]. The tungsten coatings have good adhesion with molybdenum substrate. The microstructure and crystal structure of the tungsten coating are shown in Fig. 3(a). The crystal structure of tungsten coatings is columnar crystal. From the XRD analysis in Fig. 3(b), the crystal orientation is [100]. 3.2. Thermal shock properties Tungsten coatings were not delaminated after 20 times thermal shock test. Fig. 4 shows scanning electron micrograph (SEM) of cross-sectional specimen. From
Fig. 2. Electron microprobe scanning curve in the tungsten/molybdenum diffusion layer.
D. Jihong et al. / Surface & Coatings Technology 198 (2005) 169–172
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Fig. 3. Microstructure and crystal structure of the tungsten coating: (a) crystal structure of tungsten coating and (b) XRD pattern of tungsten coating.
the result of microscopy observation, no apparent cracks were observed at the interface between the CVD layer and the substrate, which means tungsten coatings have the good adhesion with molybdenum substrate. Some pores are also found in the molybdenum substrate; this is
because the molybdenum substrate is made by powder metallurgy method. There are some pores and volatile elements in substrate, so the pores become more distinct with diffused molybdenum and with volatilization of elements.
Fig. 4. SEM image of cross sectional specimen after 20 times thermal shock.
Fig. 5. The electron microprobe scanning curve after high-temperature diffusion.
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3.3. Interface properties after high-temperature diffusion The electron microprobe scanning curve in the W/Mo diffusion layer after high-temperature diffusion is shown in Fig. 5. The mutual diffusion between the molybdenum substrate and the tungsten coating happened in the high-temperature annealing, while the thickness of the diffusion layer is 4 Am. The mutual diffusion results in the interface movement. The interface movement is not sufficient if the diffusion layer is too thick, which reduces the bonding strengthen. Oxygen, an element in the specimen which caused the surface cleaning and deposition process, will diffuse towards both sides of the interface and would cause slow deposition rate and bad adherence.
4. Conclusion (1) CVD tungsten coating has good adhesion with molybdenum substrate.
(2) Tungsten coating has good resistant thermal shock property. (3) The crystal structure of tungsten coatings is columnar crystal, and the crystal orientation is [100]. (4) High-temperature diffusion is help to increasing the adhesion.
References [1] P.A.C. Groenen, et al., Appl. Surf. Sci. 78 (1994) 123. [2] George Garnett, et al., Preparation Method for b111N preferred orientation tungsten. CN85109258A. [3] Li Hanguang, Nonferrous Metal and Hard Alloy 11 (1994) 59. [4] Yu Zongsen, Tian Zhongzuo, Met. Phys. (1982) 175.
Surface characterization of CVD tungsten coating on molybdenum substrate Du Jihong*, Li Zhengxiang, Liu Gaojian, Zhou Hui, Huang Chunliang Northwest Institute for Nonferrous Metal Research, Weiyang Road 96, Xi’an, Shannxi 710016, PR China Available online 1 January 2005
Abstract Surface characterization and microstructure studies are performed on chemical vapor deposited (CVD) tungsten coating. There is about 2-Am-thick diffusion layer of tungsten in the molybdenum substrate. The thermal shock test shows that tungsten coating has good adhesion with molybdenum substrate, but the elements of oxygen and carbon in the tungsten coating have the bad effects to the adhesion. The result of high-temperature diffusion experiment in the diffusion rate from molybdenum substrate to tungsten coating is faster. D 2004 Published by Elsevier B.V. Keywords: Chemical vapor deposited; Molybdenum substrate; Tungsten coating
1. Introduction Tungsten has good thermal conductivity and the highest melting point of all metals. These physical properties make it widely use in the electronic industry and spaceflight application. Chemical vapor deposition (CVD) is a coating method that utilizes the decomposition of a gaseous precursor, flowed over or through a heated substrate, with subsequent condensation from the vapor state to solid deposit. Tungsten film can be prepared in several ways. Evaporation, sputtering, and CVD are the most commonly used processes in industry. CVD has several advantages over evaporation and sputtering, including better step coverage and low coating time. The equipment necessary to obtain high-quality films is also relatively economical in comparison to other techniques. Additionally, tungsten can be deposited selectively on desired areas under certain conditions to remove postprocessing steps such as masking and
* Corresponding author. E-mail address: [email protected] (D. Jihong). 0257-8972/$ - see front matter D 2004 Published by Elsevier B.V. doi:10.1016/j.surfcoat.2004.10.130
etching. Tungsten deposition is generally accomplished industrially by the reduction of tungsten hexafluoride or chloride tungsten by hydrogen gas [1–3]. In the present work, the properties and the adhesion of CVD tungsten coating were studied. The interface of tungsten coated molybdenum was observed by scanning electron microscopy. Thermal shock tests were performed in order to elucidate the durability of the coated layer.
2. Experiment Schematic diagram of CVD apparatus is shown in Fig. 1. The tungsten chloride used in our experiments was produced by the chlorination of tungsten in a quartz tube which was heated in a furnace to 850–950 8C. The flow rate of gases being measured by a mass flow meter. The reaction gases were pumped away using a rotatory pump. The substrate was rotated during the deposition process to yield a deposit of uniform thickness. The samples were molybdenum tube (U20600). The surface structure elements and corresponding crystal orientations of the deposited layer was investigated
170
D. Jihong et al. / Surface & Coatings Technology 198 (2005) 169–172
3. Results and discussion 3.1. The cross-sectional microstructure and the crystal orientation of deposited tungsten layers
Fig. 1. Schematic diagram of CVD apparatus. 1-hydrogen entrance; 2chlorine entrance; 3-tungsten powders; 4-specimen; 5-induction coil; 6rotary pole; 7-waste gases exit; 8-rotary system.
using scanning electron microscope. The thermal shock properties of deposited layers are studied after 25–1400 8C cycling of 20 times. The high-temperature diffusibility of deposited layers was studied by heating at 1800 8C for 2 h.
Fig. 2 shows the electron microprobe scanning curve in the tungsten/molybdenum diffusion layer. There is a mutual diffused layer between tungsten coatings and the molybdenum substrate; the thickness of the diffusion layer is about 2 Am. The diffusion depth of molybdenum into the tungsten layer is deeper. Since molybdenum and tungsten have the same crystal structure (b.c.c.), the similar radius, and the total mutual solvability, they could mutually diffuse in hightemperature annealing. The self-diffusion activation energy of metal has constant ratio to melting point temperature and the melting point temperature of tungsten is higher than that of molybdenum, so that the self-diffusion activation energy of tungsten is higher, and the diffusion rate of molybdenum in tungsten layer is faster [4]. The tungsten coatings have good adhesion with molybdenum substrate. The microstructure and crystal structure of the tungsten coating are shown in Fig. 3(a). The crystal structure of tungsten coatings is columnar crystal. From the XRD analysis in Fig. 3(b), the crystal orientation is [100]. 3.2. Thermal shock properties Tungsten coatings were not delaminated after 20 times thermal shock test. Fig. 4 shows scanning electron micrograph (SEM) of cross-sectional specimen. From
Fig. 2. Electron microprobe scanning curve in the tungsten/molybdenum diffusion layer.
D. Jihong et al. / Surface & Coatings Technology 198 (2005) 169–172
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Fig. 3. Microstructure and crystal structure of the tungsten coating: (a) crystal structure of tungsten coating and (b) XRD pattern of tungsten coating.
the result of microscopy observation, no apparent cracks were observed at the interface between the CVD layer and the substrate, which means tungsten coatings have the good adhesion with molybdenum substrate. Some pores are also found in the molybdenum substrate; this is
because the molybdenum substrate is made by powder metallurgy method. There are some pores and volatile elements in substrate, so the pores become more distinct with diffused molybdenum and with volatilization of elements.
Fig. 4. SEM image of cross sectional specimen after 20 times thermal shock.
Fig. 5. The electron microprobe scanning curve after high-temperature diffusion.
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3.3. Interface properties after high-temperature diffusion The electron microprobe scanning curve in the W/Mo diffusion layer after high-temperature diffusion is shown in Fig. 5. The mutual diffusion between the molybdenum substrate and the tungsten coating happened in the high-temperature annealing, while the thickness of the diffusion layer is 4 Am. The mutual diffusion results in the interface movement. The interface movement is not sufficient if the diffusion layer is too thick, which reduces the bonding strengthen. Oxygen, an element in the specimen which caused the surface cleaning and deposition process, will diffuse towards both sides of the interface and would cause slow deposition rate and bad adherence.
4. Conclusion (1) CVD tungsten coating has good adhesion with molybdenum substrate.
(2) Tungsten coating has good resistant thermal shock property. (3) The crystal structure of tungsten coatings is columnar crystal, and the crystal orientation is [100]. (4) High-temperature diffusion is help to increasing the adhesion.
References [1] P.A.C. Groenen, et al., Appl. Surf. Sci. 78 (1994) 123. [2] George Garnett, et al., Preparation Method for b111N preferred orientation tungsten. CN85109258A. [3] Li Hanguang, Nonferrous Metal and Hard Alloy 11 (1994) 59. [4] Yu Zongsen, Tian Zhongzuo, Met. Phys. (1982) 175.