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Pulsed bias effect on roughness of TiO2:Nb films deposited by grid assisted magnetron sputtering
Applied Adhesion Science volume 3, Article number: 1 (2015)
In this work Nb-doped TiO2 (TiO2:Nb) films were deposited by reactive sputtering. The substrate was biased with negative pulses to change the energy of the ions nearby the sample surface during the deposition. As consequence, the film crystalline structure and roughness were changed. It was verified that higher energy favours the rutile growth with a higher roughness, even under low temperature as 300°C, and the material structure can be controlled by setting the duty cycle, voltage and frequency of the switched power supply applied to the substrate.
TiO2 films have been investigated because of their wide variety of technological applications. For example, the adhesion of the human oral mucosa soft tissue on TiO2 microimplants was evaluated in , observing some potential clinical benefits, including the possibility of bone resorption. Other examples include the TiO2 use in photovoltaics and photocatalysis fields [2-5].
Increasing the implant surface roughness enhances its adhesion to the soft tissue, but can also increase the infection risk due to the bacterial accommodation in the space between its valleys and peaks. The photocatalytic property of TiO2 in the anatase structure to eliminate bacteria was evaluated in , identifying its use as an excellent way for sterilizing the implants surfaces and reduce the infection risk. The incorporation of Nb into the TiO2 favors the nucleation of grains and electrical conductivity, producing nanostructured and transparent films [7-10].
Thin film deposition by magnetron sputtering enables the control of different parameters that can improve physical-chemical properties, microstructure and film adhesion on the substrate. The substrate negative bias is a strategy used for increasing the kinetic energy of the ions striking the film surface during the deposition . In this work, the effects of pulsed bias in the microstructure of TiO2:Nb films deposited on stainless steel substrate were analysed by X-Ray diffraction (XRD) and Atomic Force Microscopy (AFM).
The deposition chamber is a stainless steel vacuum vessel with 30 cm (diameter) by 30 cm (height). The ultimate pressure inside the chamber is 10−5 Torr, done by a vacuum system composed of a mechanical pump and a turbo molecular pump. Inside the chamber there are a magnetron, a grounded grid, a sample holder for 6 samples and an electric heater (controlled by a PID). The temperature is monitored with a thermocouple inserted in the substrate holder and its temperature is maintained at 300°C. The working pressure is measured by a capacitive gauge Adixen ASD 2004 and the base pressure is measured with a Penning gauge Edwards CP 25 EK. The gases flowing into the chamber are controlled by two mass flow meters of 200 sccm.
The base plasma is generated by a direct current power supply (up to 1000 V and 2.0 A). The (Figure 1) shows a schematic drawing of the experimental setup.
The TiO2:Nb films were deposited on stainless steel using a Grid Assisted Magnetron Sputtering System . The target used was a titanium disc (99.5% purity) with 100 mm in diameter holding 12 inserts of niobium (99.5%) with 3 mm in diameter each one, disposed in the erosion area, producing Ti/Nb films with ratio of 9:1. The working gas used in the sputtering process was a mixture of Ar and O2 in order to get a TiO2:Nb film - experimental details are showed in (Table 1).
The waveform of substrate bias was acquired using an oscilloscope Tektronix TDS 2024B. The (Figure 2) shows an example of the substrate voltage and current signals during the film deposition. The time “on” divided by the period is called “duty cycle” (t/T as showed in (Figure 2)). It means the percentage of time, during one period, on which the substrate is biased. The pulsed power supply is a homemade device specially developed for this work.
The duty cycle was varied from 30% up to 70%. The peak substrate bias was -100 V.
The (Table 1) shows the summary of the experimental parameters set to the experiment.
The crystalline planes were identified by the SHIMADZU XRD-6000 equipment, using 2θ diffraction angle, with radiation Cu Kα 1,5406 Å.
The chemical composition and also the film thickness were measured by energy dispersive X-ray fluorescence spectrometer (EDX), SHIMADZU EDX-720 model, adjusted in the fp thin films mode to get the signal from a solid state X-ray detector, which was in good agreement with confocal measurements. Follow below the procedure used:
Measure the raw substrate in order to identify its own composition;
Insert the composition data about the raw material for thickness identification using the fp thin films mode and measure some samples to verify the accuracy, comparing the results with the preview confocal measures;
Measure the samples using the bulk fp method for wavelength-dispersive spectrometers.
It is important to clarify that the equipment used does not measure small atoms like oxygen.
For the characterization of the surface morphology and roughness in nanometer scale was used a NANOSURF AFM NANITE B S200 equipment, adjusted in the dynamic force mode (constant height).
The average films thickness measured by EDX was (0.28 ± 0.05) μm and its chemical composition is showed in Table 2.
Results and discussion
The (Figure 3) shows the XRD pattern for films deposited at different duty cycles. It is observed that the rutile peak (110) has a strong increase in intensity with increasing duty cycle from 30% up to 70%. Besides that, others rutile peaks of very low intensity (101), (200), (111), (211) and (220) are also identified. The (111) and (200) peaks correspond to the iron alpha phase (α-Fe) of the stainless steel substrate. For 30% duty cycle there is also a shoulder indicating the presence of (101) anatase (25°) in green.
According to (Figure 3), depending on the duty cycle adjustment, the TiO2 nucleation can favor the rutile crystalline structures.
The (Figure 4) shows the AFM images for the films deposited at different duty cycles. The grain size increases with increased duty cycle, as well the average surface roughness.
The (Table 2) shows a summary of roughness and grain size measurements. The grain size was calculated by CTR AFM software, using the surface profile (dark for valleys and bright for peaks and vice-versa), and the average value was calculated for each condition (b), (c) and (d). Similarly, the average surface roughness “Sa” was calculated using (eq. 1) taking as reference the AFM NANITE B S200 manual.
where, z (xk,xl) is the peak intensity for each (k,l) point.
By comparing the AFM and XRD results it is possible to correlate the crystallinity and roughness: both increased with the duty cycle. The increased duty cycle means a larger bombardment time and as consequence promotes higher energy delivered to the film.
The results showed that the pulsed bias can produce significantly changes on the roughness and the growth of crystalline phases in the film: the rutile phase and roughness are favoured when the substrate is subjected to a higher energy bombardment. For the conditions described in this work, the TiO2:Nb film formed is rather rutile with preferencial growth (110).
It is observed that increasing the duty cycle from 30% up to 70% (for peak Vbias = −100 V and 100 kHz) increases the intensity of the rutile (110) as well the nanometric grain size of the film deposited. It is also verified that the average surface roughness increases almost three times by increasing the duty cycle from 30% up to 70%.
Finally, based on the results observed, it is possible to conclude that by setting the duty cycle of the pulsed voltage applied to the substrate allows the control of the film roughness and grain size that can be used, for example, for potential benefits at adhesion clinical applications.
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The authors declare that they have no competing interests.
AR was responsible for the AFM measurements. JSS was responsible for the homemade switching power supply and experiments. JS was responsible for the films deposition. JCS and LCF were responsible for the results discussions. MM was responsible for supervising. All authors read and approved the final manuscript.
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Scholtz, J.S., Stryhalski, J., Sagás, J.C. et al. Pulsed bias effect on roughness of TiO2:Nb films deposited by grid assisted magnetron sputtering. Appl Adhes Sci 3, 1 (2015) doi:10.1186/s40563-015-0031-7
- Nb-doped TiO2 films
- Reactive deposition
- Substrate bias
- Switched power supply