Nanomechanical characteristics of SnO
2
:F thin films
deposited by chemical vapor deposition
Te-Hua Fang
a, Win-Jin Chang
b,*
a
Department of Mechanical Engineering, Southern Taiwan University of Technology, Tainan 710, Taiwan bDepartment of Mechanical Engineering, Kun Shan University of Technology, 949 Tawan Road, Tainan 710, Taiwan
Received 28 January 2005; received in revised form 16 March 2005; accepted 19 March 2005 Available online 15 April 2005
Abstract
The nanoindentation characterizations and mechanical properties of fluorine-doped tin oxide (SnO2:F) films deposited on
glass substrates, using chemical vapor deposition (CVD) method, were studied, which included the effects of the indentation loads, the loading time and the hold time on the thin film. The surface roughness, fractal dimension and frictional coefficient were also studied by varying the freon flow rates. X-ray diffraction (XRD), atomic force microscopy (AFM) and frictional force microscopy (FFM) were used to analyze the morphology of the microstructure. The results showed that crystalline structure of the film had a high intensity (1 1 0) peak orientation, especially at a low freon flow rate. According to the nanoindentation records, the Young’s modulus ranged from 62.4 to 75.1 GPa and the hardness ranged from 5.1 to 9.9 GPa at a freon flow rate of 8000 sccm. The changes in measured properties were due to changing loading rate.
# 2005 Elsevier B.V. All rights reserved.
Keywords: Chemical vapor deposition; Thin films; Tin oxide; Nanoindentation
1. Introduction
Tin oxides are known for chemical durability, thermal stability, high mechanical properties, high reflectivity within the infrared region, and high transparency within the visible part of the spectrum; and they can be utilized in many industrial applications, for example, conductive electrodes in thin film solar cells and several optoelectronic devices [1–6]. Tin oxide is a wide band gap electron degenerate
semiconductor. Usually, fluorine or antimony is used as a dopant to improve the electro-optical properties[7]. Various methods can be used to prepare tin oxide film on many kinds of substrates, for example, reactive sputtering [8], pulsed laser deposition (PLD) [9],
electron cyclotron resonance sputtering [10] and
chemical vapor deposition (CVD) [11]. The CVD
method is usually used due to the advantages of simplicity and a higher deposition rate at a lower cost. However, when a film is deposited by CVD, it is difficult to ensure the films structural uniformity and the stability of its electrical and optical properties. The freon flow rate during the depositing process plays an
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* Corresponding author. Tel.: +886 2724833; fax: +886 2724833. E-mail address: [email protected] (W.-J. Chang).
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important role in improving the physical properties of the film.
Nowadays, nanoindentation has been employed extensively to study the mechanical properties of thin films. Woirgard et al.[12]studied the elastic modulus and the hardness of ceramic materials using nanoin-dentation technique. Beake and Leggett[13]utilized nanoindentation technique to study the mechanical properties of polymeric materials on a nanoscale.
Recently, Fang and Chang[14]studied the
nanome-chanical properties of copper thin films on different substrates using this nanoindentation technique.
In this paper, the nanoindentation behavior of SnO2:F thin films deposited on glass substrate using
the CVD technique at different freon flow rates was investigated. The microstructural properties of the films were investigated by X-ray diffraction (XRD), atomic force microscopy (AFM) and frictional force microscopy (FFM).
2. Experimental details
SnO2:F films were prepared by chemical vapor
deposition technique using 10 g SnCl45H2O and 1 g
NH4F dissolved in 120 ml H2O. Fluorine was the
doping agent using nitrogen as the carrier gas [15]. The flow rates of the gases were controlled by a mass flow controller. The pre-cleaned substrate Corning
glass 1737, 1.5 cm 1.5 cm, was mounted on an
aluminum holder inside the reaction chamber. The partial pressure of each source was controlled by adjusting the amount of carrier gas N2passed through
the bubblers with a mass flow controller. A heating string kept the outlet tubes from the bubbler at a temperature of approximately 120 8C to prevent the reaction gas from condensing during the flow.
Optimum film quality was observed at a deposition
temperature of 600 8C. After the SnO2 films were
deposited onto the substrates, they were allowed to cool down to room temperature. The films growth conditions are shown inTable 1. In order to identify the effects of the different freon flow rates on the surface morphology of the films, the rates of 2000,
4000, 8000 and 12,000 sccm (cm3/min) were used in
these experiments.
X-ray diffraction analysis of the tin oxide films was carried out using a Rigaku D/MAX-2500
diffracto-meter to investigate the structure. Cu Ka radiation with a wavelength of 0.154 nm was used as the X-ray source. The surface morphology of the films was observed with atomic force microscopy and frictional force microscopy (CP-R SPM, Veeco/TM, USA). A constant scan speed of 1 mm/s was used and a constant load was applied to the cantilever. The
measurement tip is made of Si3N4 with a 0.16 N/m
stiffness cantilever. With the aid of AFM, both the root-mean-square (RMS) and the average roughness (Ra) values of the surface were calculated in order to
study the effect of the freon flow rate. The topographic
measurement was performed on a 2 mm 2 mm area.
Nanomechanical properties, such as Young’s modulus and hardness, were obtained by nanoinden-tation (Triboscope, Hysitron, USA). The diamond indenter was a Berkovich tip with a tip radius of 100 nm[14]. Nanoindentation experiments, including the load–unload test, the creep test and the loading rate test, were performed to understand the nanomecha-nical properties of SnO2:F thin films. For the load–
unload test, the loads ranged from 1000 to 7000 mN with a loading time of 5 s, a hold time of 2 s and a freon flow rate of 8000 sccm. Hold time creep behavior experiments were performed using hold times of 1, 10, 30 and 50 s at the peak of the load and kept at a constant load of 1000 mN at a loading time of 5 s and a freon flow rate of 8000 sccm. For the loading rate tests, the loading times ranged from 10 to 50 s at a constant load of 1000 mN and a hold time of 2 s and a freon flow rate of 8000 sccm.
3. Results and discussion
The XRD patterns of the fluorine-doped tin oxide films deposited at a substrate temperature of 600 8C at different freon flow rates are shown inFig. 1. The films
T.-H. Fang, W.-J. Chang / Applied Surface Science 252 (2005) 1863–1869 1864
Table 1
Deposition parameters of stannum oxide films
Thin films SnO2:F
Substrate Glass Temperature (8C) 600 Gases (sccm) SnCl4 5000 H2O 5000 Freon 2000–12000
(1) The fractal dimension (Ds) of the film decreased
as the freon flow rate was increased, while the
average surface roughness (Ra), the
root-mean-square surface roughness (RMS) reached a constant value at higher freon flow rates. (2) The frictional coefficient or the frictional force
decreases as the freon flow rate was increased. (3) In depth-sensing indentation results, Young’s
modulus ranged from 62.4 to 75.1 GPa and the hardness ranged from 5.1 to 9.9 GPa at a freon flow rate of 8000 sccm. The changes in nano-mechanical properties were due to changing loading rate.
Acknowledgments
The authors would like to thank Dr. M.J. Chiang for his technical support. This work was partially supported by the National Science Council of Taiwan,
under Grant Nos. NSC91-2218-E218-001 and
NSC93-2212-E-218-005.
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