AIP Advances 6, 045206 (2016); https://doi.org/10.1063/1.4947091 6, 045206
© 2016 Author(s).
Study on the effect of hydrogen addition on the variation of plasma parameters of argon-oxygen magnetron glow discharge for synthesis of TiO 2 films
Cite as: AIP Advances 6, 045206 (2016); https://doi.org/10.1063/1.4947091
Submitted: 25 January 2016 . Accepted: 05 April 2016 . Published Online: 13 April 2016
Partha Saikia, Bipul Kumar Saikia, and Heman Bhuyan COLLECTIONS
Paper published as part of the special topic on Chemical Physics, Energy, Fluids and Plasmas, Materials Science and Mathematical Physics
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Study on the effect of hydrogen addition on the variation of plasma parameters of argon-oxygen magnetron glow discharge for synthesis of TiO
2films
Partha Saikia,1,2,aBipul Kumar Saikia,1and Heman Bhuyan2
1Centre of Plasma Physics, Institute for Plasma Research, Nazirakhat, Sonapur-782 402, Kamrup, Assam, India
2Institute of Physics, Pontificia Universidad Católica de Chile, Av. Vicuña Mackenna 4860, Santiago, Chile
(Received 25 January 2016; accepted 5 April 2016; published online 13 April 2016)
We report the effect of hydrogen addition on plasma parameters of argon-oxygen magnetron glow discharge plasma in the synthesis of H-doped TiO2 films. The parameters of the hydrogen-added Ar/O2 plasma influence the properties and the structural phases of the deposited TiO2 film. Therefore, the variation of plasma parameters such as electron temperature (Te), electron density (ne), ion density (ni), degree of ionization of Ar and degree of dissociation of H2 as a function of hydrogen content in the discharge is studied. Langmuir probe and Optical emission spectroscopy are used to characterize the plasma. On the basis of the different reactions in the gas phase of the magnetron discharge, the variation of plasma param- eters and sputtering rate are explained. It is observed that the electron and heavy ion density decline with gradual addition of hydrogen in the discharge. Hydrogen addition significantly changes the degree of ionization of Ar which influences the structural phases of the TiO2film. C2016 Author(s). All article content, except where otherwise noted, is licensed under a Creative Commons Attribution (CC BY) license (http://creativecommons.org/licenses/by/4.0/).[http://dx.doi.org/10.1063/1.4947091]
I. INTRODUCTION
Titanium dioxide (TiO2) thin films have been widely investigated for its various interesting properties such as photo-stability, chemical inertness, and non-toxicity.1,2TiO2based materials are considered as superior photocatalyst.3 Due to its excellent optical transmittance, high refractive index, and durability, TiO2are extensively used for optical coating.4–6TiO2also shows good adsorp- tion of ammonia, nitric oxides and many organic compounds such as hydrocarbonic and aromatic gases, alcohol and many others.7–9Therefore TiO2coatings are also suitable for gas sensor applica- tion. It is an excellent material not only for many solid-state sensors, but also for solar cells, various optical devices and many others applications.10
TiO2 film can be prepared by various deposition methods.11–13Reactive sputtering is one of the most promising methods among these because the stoichiometry of the deposited film can be controlled by varying the deposition conditions.14Magnetron sputtering has become one of the most widely used techniques for thin film deposition, including TiO2, due to its low cost and control over the film thickness, composition and structure. Recently, a new trend of codoping with nonmetals such as hydrogen and nitrogen during the deposition of TiO2by various plasma based methods is developed.15,16 The H-doping and N-doping on the TiO2thin film reduce the band gap energy of TiO2and improve the photo-catalytic activity.17The band gap narrowing of the TiO2is a result of mixing of N acceptor states with the O 2p states of TiO2which promotes the visible light driven photocatalytic activity in the case of the N-doped films. On the other hand, the introduction of
aElectronic mail:[email protected]
2158-3226/2016/6(4)/045206/9 6, 045206-1 © Author(s) 2016.
045206-2 Saikia, Saikia, and Bhuyan AIP Advances 6, 045206 (2016)
disordered phase on the surface of the nanocrystalline TiO2is responsible for the improved pho- tocatalytic activity in the case of H-doped films.17Considering the recent importance of codoping, it would be interesting to investigate the effect of hydrogen addition on the structural phase of the magnetron plasma deposited TiO2thin film. The Ar/O2magnetron discharge plasma with the addition of hydrogen or nitrogen is a multi-component plasma consisting of positive ions, negative ions and electrons.18 The behavior of plasma parameters in the background of multi-component plasma is completely different from the normal single component plasma. It is due to the complex discharge behavior of the molecular gases and complexity generated by the various plasma species under the influence of crossed magnetic and electric fields.19The plasma parameters in turn influ- ence the properties of the deposited film. Therefore the delicate dependence of plasma parameters (density profile, temperature and energy distributions) on the partial pressure of the dopant gas is undoubtedly a subject for investigation. Considering the above facts, this paper reports the variation of plasma parameters as a function of hydrogen addition in the Ar/O2magnetron discharge and the correlation of the plasma parameters with the phases of magnetron plasma deposited TiO2thin film by X Ray Diffraction (XRD) method.
II. EXPERIMENTAL SETUP
A cylindrical vacuum chamber, made of stainless steel 304 mounted with a planar magnetron was used as the deposition setup. The diameter of the chamber was 350 mm and the length was 400 mm. The magnetron that was used to deposit TiO2film is a ‘Type 2 unbalanced magnetron’. An unbalanced magnetron is intentionally configured with magnetic pieces or coils that surround the central pole piece magnet. The central pole piece has been made much stronger than the perimeter pole piece, resulting in an additional axial magnetic field. In this case, some of the field lines are directed towards the substrate. Consequently, the plasma is no longer strongly confined to the target region but is allowed to flow out towards the substrate. A water-cooled Ti target of diameter 70 mm was used as the cathode of the planar magnetron sputtering system. The schematic diagram of the experimental set-up is shown in Fig.1.
For the deposition of TiO2on mirror-polished Si (100) substrate, Argon (Ar) and Oxygen (O2) were fed into the deposition chamber regulated by digital mass flow controllers (DFC 26, AAL- BORG USA). A fixed argon and oxygen partial pressure of 0.30 Pa and 0.05 Pa was maintained in all the deposition conditions. Hydrogen was then added gradually due to which the working pressure increased from 3.5 × 10−1Pa (without hydrogen) up to 6.0 × 10−1Pa (See Table I). The deposition pressure was monitored using a capacitance manometer (CMR 374, Pfeiffer Vacuum, Germany) with a working pressure range of 1.1mbar to 10−4mbar. With the addition of hydrogen to
FIG. 1. Schematic diagram of the experimental set-up.
TABLE I. Values of partial pressures and flow rates of different gases at various discharge conditions.
Gas type
Argon Nitrogen Hydrogen
Pressure Flow rates Pressure Flow rates Pressure
Total pressure
(X 10−1Pa) (X 10−1Pa) (SCCM) (X 10−1Pa) (SCCM) (X 10−1Pa)
3.5 3.0 16 0.5 2.0 0
4.0 3.0 16 0.5 2.0 0.5
4.5 3.0 16 0.5 2.0 1.0
6.0 3.0 16 0.5 2.0 2.5
SCCM refers to Standard Cubic Centimeters per Minute.
oxygen added argon plasma, a decrease of discharge current from 0.20 A to 0.179 A was observed at a fixed input power of 100W (Table II). For processes operated at a constant power, the obser- vation of an increase in target voltage must either be accompanied by a corresponding decrease in the ion current or in the secondary electron coefficient (γse). Because the change in the secondary electron coefficient (γse) is not significant as a function of working pressure, the change in voltage was compensated by a corresponding change in the ion current, which is related to the discharge current by20
Id= (1 + γse)Ii (1)
For most metals γse∼ 0.05–0.2 for typical ion energies encountered in magnetron sputtering.
Hence, the dominating fraction of the discharge current at the target is the ion current and it de- creases as a function of hydrogen addition.21The plasma parameters like electron temperature (Tein eV), electron density (ne) and ion density (ni) were measured by a cylindrical Langmuir probe made of tungsten having a radius of 0.15 mm and length of 3 mm. After every set of measurements, the probe tip was cleaned to ensure that the charge collection area of the probe is invariant in different deposition conditions. Langmuir probe measurements were taken at the center of the discharge in the downstream region 80 mm from the cathode, which is 10 mm above the substrate. The probe measurements were taken near the substrate plane, keeping in mind that the electron and ion density in the vicinity of the substrate can significantly affect the structural phases of the deposited TiO2
thin film. The relative concentration of ions in the plasma is estimated by using a 1/2 m digikrom spectrometer (CVI Laser Corp, USA. Digikrom Model DK 480). The optical system consists of a photo multiplier tube (PMT: Model AD110, wavelength range: 185 – 930nm) and a grating with 1200 grooves/mm for detection in the region λ = (300 – 950) nm. The entrance slit and exit slit of the monochromator were adjusted at 20 Å to obtain an acceptable spectral resolution sufficient to detect the emission lines. The emissions from the discharge were collected by a light collecting system (LCS) through optical fiber (F) connected to the spectrometer. The variations in the intensity of different species are observed with the LCSF placed at a vertical distance of 80 mm below the cathode surface. The LCSF comprised of a plano-convex lens (diameter 2 mm, focal length 3 cm) and a silica fiber of 1.0 mm core (numerical aperture: 0.22). The view subtended by this LCS was parallel to the discharge column grazing the vicinity of the substrate and always collected radiations
TABLE II. Values of discharge voltage (V) and current (A) at various hydrogen partial pressures.
H2partial pressure (×10−1Pa.) Voltage (V) Current (mA)
0 500 200
0.5 537 186
1.0 550 180
2.5 558 179
045206-4 Saikia, Saikia, and Bhuyan AIP Advances 6, 045206 (2016)
from the fixed location. A Crystal Thickness Monitor (DTM-101) manufactured by HIND-HIVAC, INDIA is used to monitor the deposition rate at the various discharge conditions. The monitor head was placed at the same horizontal plane as the substrate holder. The substrate temperature was monitored by a thermocouple fixed on the backside of the substrate holder and kept at 300oC during deposition.
III. RESULTS AND DISCUSSION
A. Influence of hydrogen addition on the current-voltage curve of the magnetron target The behavior of the magnetron target due to addition of reactive gases changes during the magnetron operation. The usual procedure to investigate the influence of the target material on the evaluation of discharge parameters is to study the current-voltage curve of the magnetron target.
A detailed investigation of the effect of hydrogen addition on the current-voltage curve of the magnetron target is presented in this section. The current-voltage curve of the magnetron discharge has been proposed by Westwood et al.22as
I = β(V − V0)2 (2)
with V0is minimum voltage required to maintain the discharge, β is a constant which is a measure of the steepness of current-voltage curve. Value of V0 and β can be determined by a linear fit to square root of current (I) as a function of voltage. It has been shown by D. Depla et al.23 that β increases with increasing ISEE coefficient values. Based on Eq. (2) a linear behavior is expected when square root of the current is plotted as a function of discharge voltage. The magnetron current-voltage curves were determined at different deposition conditions and are shown in the Fig. 2(a). The initial chemisorptions occurring on the target surface of Titanium leads to linear behavior of current versus voltage only at sufficiently high value of discharge current. The corre- sponding values of β are evaluated and are plotted as a function of working pressure in Fig.2(b).
However, it is found that the change of β as a function of hydrogen addition to the discharge is not significant. As such, it is concluded that the impact of target property on the evaluation of plasma parameters is not considerable in the present investigation.
B. Langmuir probe and Optical emission spectroscopy study
As hydrogen is added to the Ar/O2magnetron discharge, the plasma parameters in the discharge begin to change. The electron density and electron temperature are determined from the I-V characteristics of the Langmuir probe as a function of hydrogen partial pressure. Addition of hydrogen to oxygen containing argon discharge leads to a continuous decrease of electron density and a corresponding increase of electron temperature from 6.30 to 6.74eV (see Fig.3). It is because of the fact that with the addition of hydrogen to the oxygen added argon discharge, several additional loss channels of electrons appear. It is interesting to mention here that the increase in electron temperature can be accounted for the reduction of the value of electron density in the discharge. As hydrogen added Ar/O2plasma contains various ion species (Ar+, ArH+, O+, O2+, O−, H+, H2+, H3+ etc.), it is not possible to determine the density of each ion individually using the Langmuir probe.
Also, due to the presence of these ionic species the use of argon ion mass in the calculation for ion density is not valid. Thus rather than calculating the density of each ion, group density is calculated following the mean ion mass procedure as developed by Laidani et al.24For that purpose, we have divided the ions present in such discharge into two groups. A heavy group of mass 37.67 amu consists of Ar+, ArH+and O2+ions and a light group of mass 2.0 amu consists of hydrogen-like ions (H+, H2+, and H3+ ions). Assuming quasi neutrality of the plasma, the value of mean ion mass for the composite plasma is easily determined, once we find out the value of electron density from the electron saturation part of the I-V characteristics. After computing the value of mean ion mass, the heavy ion density is easily calculated by taking the ratios of ion saturation current of the composite plasma to pristine argon plasma at similar deposition conditions. It is interesting to note that while heavy ion density in the discharge is inversely proportional to the hydrogen enrichment
FIG. 2. (a) I –V characteristics for magnetron target material on a√
I −V plot (b) The influence of hydrogen partial pressure on β.
in the feeding gas, the light ion density scales linearly with it (see Fig.4). As the difference between the masses of the two ion groups is large therefore the error in calculating the plasma parameters from the I-V characteristics is not significant. This was of great help in deriving the composition of the ionic part of the plasma and in studying the effect of hydrogen on it. In fact, the density variation of heavy and light ion mainly signifies the variation of Ar+and H2+ion densities in the discharge.
In the Fig.5intensity variation of the species of Ar I [3s23p5(2p3/20 ) 4s]→ [3s23p5(2p1/20 ) 4p]
at 696.45 nm, 738.39 nm, 801.47 nm, 811.53 nm; Ar I [3s23p5(2p1/20 ) 4s]→ [3s23p5(2p1/20 ) 4p]
at 751.46 nm, 763.51 nm, 772.37 nm, 794.81 nm, 826.45 nm, 840.42 nm, 852.14 nm; Ar II [3s2 3p4(3p)4s]→ [3s23p4(3p)4p] at 434.80 nm, 454.15 nm, 472.68 nm, 476.48 nm, 480.60 nm; Hαline (656.28 nm) and Hβline (487.54 nm) and O I [2s22p3(4S0)3s] → [2s22p3(4S0)3p] at 777 nm and 845 nm are shown.25,26To a good approximation in low pressure discharge, the emission intensity of a particular line of an element is considered to be proportional to the density of that particular species.27,28Comparing the line intensity ratio of selected transitions one can estimate the degree of dissociation and ionization of particular species present in the discharge. It have been reported that the line intensity ratios of IH/IArcan be used to determine the degree of dissociation in hydrogen
045206-6 Saikia, Saikia, and Bhuyan AIP Advances 6, 045206 (2016)
FIG. 3. Modulation of electron density and electron temperature as a function of H2partial pressure.
plasma using the following relation:29,30
NH = kIH
IAr
NArwith k= αArAArτArλnl αHAnnτnnλnn
(3) where αH,Ar are the rate coefficients for direct excitation of the corresponding Ar and H atom levels, AAr,nnand τAr,nnare the radiation transition probabilities and life times of the corresponding excited states respectively, and λ is the wavelength of transition. In the present work, we have chosen the ratio of Hαto Ar-811.53 nm lines to estimate the degree of dissociation as the excitation thresholds for such transitions are very close and the transition probabilities are almost similar.31,32 With the addition of hydrogen, the intensity ratio of the lines of excited species of Hα(656.28 nm) to Ar (811.53 nm) increases (Fig.6). It indicates the increase of degree of dissociation of hydrogen molecule. As the electron impact dissociation of molecular hydrogen is the dominant production scheme of H atom, this behavior is not surprising.33 Excited neutral hydrogen atoms (H with n
= 2) in such plasma are produced by various recombination processes of hydrogen-like ions (Eqs. (4) and (5)).34
e−+ H2+→ H+ H (4)
e−+ H3+→ H+ H + H (5)
FIG. 4. Influence of H2partial pressure on the variation of the heavy and light ion density.
FIG. 5. (a) OES pattern of the hydrogen added Ar/O2magnetron discharge at different hydrogen partial pressures. (b) View of selected transitions is shown in this figure.
Thus, H (n= 2) characteristic emission line at 656.28 nm could also be used as a representative of relative density of hydrogen-like ions in such plasma. The gradual increase of Hα(656.28 nm) to Ar (811.53 nm) also indicates the increase of hydrogen-like ions with increasing hydrogen partial pressure. The introduction of hydrogen can also affect the degree of ionization of argon in the discharge. The intensity ratio of Ar (II) at 476.48 nm to Ar (I) at 751.46 is the representative of the degree of ionization of argon and is found to decrease with the introduction of hydrogen in the discharge (Fig.7).
C. Effect of hydrogen addition on the sputtering yield of target
The addition of hydrogen has a significant effect on the sputtering yield of titanium target. It is found that as the hydrogen content in the discharge is increased, the sputtering rate decreases from 1.8 A0s−1at working pressure of 3.5 × 10−1Pa to 1.5 A0s−1at working pressure of 6 × 10−1Pa. As the sputtering yield of titanium target due to Ar+ions is greater than the other positive ions present in the discharge, therefore higher value of deposition rate is obtained at lower hydrogen partial
FIG. 6. Influence of the H2partial pressure upon the intensity (I) ratio of the hydrogen atomic line at 656.28 nm (Hα) to argon atomic line at 811.53 nm.
045206-8 Saikia, Saikia, and Bhuyan AIP Advances 6, 045206 (2016)
FIG. 7. Variation of the intensity (I) ratio of the intensity (I) ratio of the 476.4 nm argon line (Ar II) to argon atomic line at 751.46 nm.
pressure.23Gradual increase in hydrogen partial pressure reduces the value of the Ar+ion density and as such, the deposition rate ofTiO2thin film.
D. Effect of hydrogen addition on the phases of the deposited film
Fig.8 shows the XRD pattern of the films deposited at different hydrogen partial pressures.
Since titanium oxide films are deposited without any substrate heating, only a sharp diffraction peak that can be assigned to the anataseTiO2phase (211) with tetragonal structure (ICDD: 00-021-1272) is observed.35It is observed from the XRD figure that with the increase in hydrogen partial pressure, the intensity of the anatase phase decreases gradually. Although roughly the same thickness thin films are deposited, the decrease in the (211) peak intensity of theTiO2 film with the hydrogen addition is due to the decrease of degree of bombardment on the substrate.14This observation is supported by the facts that the ion current, sputtering rate and argon like ion density decrease with the addition of hydrogen. It decreases the total energy of the particles arriving at the substrate sur- face. Also due to increase of pressure the energy of particles arriving at substrate surface decrease,
FIG. 8. The X.R.D pattern of the films deposited at different hydrogen partial pressures.
leaving less energy for surface diffusion.14It will result a decrease of surface mobility and therefore, a variation in the phase structure of the deposited thin films.
IV. CONCLUSION
In conclusion, the paper reports that with introduction of hydrogen in the Ar/O2 magnetron discharge plasma, the electron density decreases from 7.5 × 1015 m−3 to 6.4 × 1015 m−3 with a corresponding increase of electron temperature from 5.9 eV to 6.54 eV. A decrease of total heavy ion (Ar+, ArH+ and O2+) density and increase of light ion density (H+, H2+, and H3+ ions) are also observed upon hydrogen addition to the magnetron discharge. The decrease of degree of ioni- zation of argon is found to be responsible for the decrease of deposition rate of TiO2 at higher hydrogen partial pressure. In addition, an increase in degree of dissociation of hydrogen molecule plus the hydrogen like ion density with increasing hydrogen content in the discharge is also found responsible for the observed structural change. Thus, a correlation between the magnetron plasma parameters with the structural phase of deposited TiO2thin film is observed.
ACKNOWLEDGEMENTS
Authors acknowledge FONDECYT grant 1130228 and 3160179. Additional funding from Conicyt PIA program ACT1108 is also acknowledged.
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