Pulsed bias effect on roughness of TiO2:Nb films deposited by grid assisted magnetron sputtering
© Scholtz et al.; licensee Springer. 2015
Received: 20 November 2014
Accepted: 8 January 2015
Published: 11 February 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.
KeywordsNb-doped TiO2 films Roughness Reactive deposition Substrate bias Switched power supply
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.
Summary of the experimental parameters
Oxygen mass flow rate
Argon mass flow rate
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.
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).
Summary of the experimental results for films deposited with V bias = −100 V pulsed at 100 kHz
Av. grain size
Av. surface roughness (Sa)
(a) raw substrate
Results and discussion
According to (Figure 3), depending on the duty cycle adjustment, the TiO2 nucleation can favor the rutile crystalline structures.
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.
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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