Nanometric thin films have always been chiefly used for decoration; however they are now being widely used as the basis of high technology. Among the various physical qualities that characterize them, the thickness strongly influences their properties. Thus, a new procedure is hereby proposed and developed for determining the thickness of V2O5 nanometric thin films deposited on the glass surface using Portable X-Ray Fluorescence (PXRF) equipment and the attenuation of the radiation intensity K
Thin films have always been intended to be utilized primarily for decorative purposes; however they are currently being used for multiple purposes. It has contributed to the technological revolution of integrated circuits and telecommunications [
Moreover, when physical and chemical techniques are combined, thin films can be produced from any material including organic or inorganic, metals, metal oxides, and polymers [
Thin films are layers whose thickness is naturally decisive for their electromagnetic and quantum characteristics [
The thickness measurement of materials, especially thin films, is currently being made by sophisticated and very expensive devices which often need a proper sample treatment to perform the desired measurement.
Among those techniques, the Portable X-Ray Fluorescence (PXRF) presents a low cost that dispenses sample preparations. Furthermore, it can be installed for
Vanadium oxide thin films were deposited by resistive thermal evaporation in high vacuum. Glass plates as substrates were used for both microscope and X-rays fluorescence measurements. The substrates were cleaned in ultrasonic acetone baths solutions followed by isopropanol, which were subsequently dried in hot plate to complete the evaporation of solvents and water. Vanadium oxide films of different thicknesses were deposited with an amount variation of V2O5 powder mass, Figure
V2O5 thin films were used in the calibration techniques with 160, 110, and 48 nm of thickness, respectively, from left to right, measured by SEM.
The evaporation bath containing the oxide to be evaporated in tablet form is made of metallic tungsten. The material in tablet form was obtained from uniaxial quantities of 130 mg, 100 mg, 55 mg, 45 mg, and 35 mg from V2O5 powder (Sigma-Aldrich, 99.9%) weighted on an analytical scale. The film depositions were done in a brand system called HHV (Auto 306). The substrates were held in a rotated substrate holder to ensure a better uniformity in the film thickness during the deposition.
The substrate holder and the substrates were kept in room temperature. A glow-discharge process was performed to ensure the cleanliness of the substrates before the deposition. The back pressure was 7.89 × 10−6 mbar and 1.15 × 10−5 mbar during the deposition.
In the Scanning Electron Microscopy (SEM) an electron beam is accelerated with a potential difference of thousands of volts; when it reaches the sample, it is reflected and directed to the detector. In case of nonconductive samples as the studied films, a thin film conductor ought to be deposited on its surface so that an electric charge of the surface does not occur; this could lead to an electrostatic deviation of the incident electrons.
A SEM was employed to determine the thin films thickness from 100 mg, 70 mg, and 35 mg of V2O5 powder; this thickness is subsequently confronted with the Portable X-Ray Fluorescence technique average thickness. A FEI Quanta 200 microscope (Oregon, USA) which belongs to the University of Londrina Electron Microscopy Laboratory and Microanalysis was used. The images were performed employing an acceleration voltage of 30 kV.
A
A MAGNUM MUHV Mini X-ray tube manufactured by Moxtec (Moxtec Inc.) with both silver target (Ag) electric current and electric voltage is controlled by a high-voltage source. This tube can be operated in up to 40 kV and 100
For a better system performance Ag filter with 100
Therefore, the use of the Ag filter provides a better value for the peak/background ratio, allowing an accurate analysis of the elements in the energy range of 3–15 keV [
An XR-100CR Si-PIN detector with a preamplifier, a thermoelectric cooling system, a conjugated high-tension source module, and an amplifier was also employed. This detector has 6 mm2 crystal by 500
X-rays from the samples were collimated using a 1 cm long by 0.5 mm diameter Ag cylinder. This collimator was used to minimize the dead time in the measurement and the Compton scattering. Both the detector and the tube were positioned at 45° on the samples surface.
The measurements were performed with the samples 2 cm away from the tube and 2.5 cm from the detector; as a result the analyzed area was approximately 0.8 cm2. The excitation and detection time for each measurement was 300 seconds. Each sample was analyzed in triplicate.
The spectral analyses were performed using WinQXAS software. The whole system is shown in Figure
Portable X-Ray Fluorescence system: (1) sample support, (2) X-ray minitube, (3) X-ray detector, (4) standard electronic (5) laptop computer.
When the X-ray photons go through the matter, the emerging beam intensity (
If we consider a simple case of a thin film of vanadium deposited on a glass flat surface (substrate), the film thickness could be determined by the measurement of a constituent element attenuation intensity of the substrate [
Critical analyses of five substrates were previously performed to identify the main constituent elements of the substrate (glass) and then determine which element had the best peak/background (P/B) ratio and also the best resolution (FWHM). An illustrative spectrum is presented in Figure
PXRF spectra obtained for the substrate. Measurement time 300 s.
As it can be seen in Figure
Average values for the ratio peak/background and FWHM for the glass substrate number 5.
Element | Energy (keV) | Peak/background | FWHM | |
---|---|---|---|---|
(Channel) | (keV) | |||
Fe | 6.40 | 1.15 | 4.99 | 0.14 |
Ca | 3.69 | 1.07 | 4.12 | 0.11 |
Based on the results shown in Table
Thus, assuming that a thin film with a nanometric vanadium thickness is deposited on a substrate of another element (Ca), the measured intensity for the X-rays in which characteristic of the Ca-K
Theoretical attenuation according to (
Theoretical attenuation according to (
Despite the fact that the attenuation of the radiation by the matter is exponential, for sub-micrometers thicknesses (
To determine the thickness of the vanadium film in unknown samples using PXRF with the proposed theoretical model a calibration method for the portable spectrometer is required. The calibration method proposed was made with a series of standard samples with known thicknesses where a net area curve (counts) under the peak calcium (K
Three samples of vanadium film, V2O5, with known masses were deposited on a glass substrate as described in Section
Thicknesses of deposited films in terms of material mass that has been used for each evaporation.
Mass (mg) | Thickness measured by SEM (nm) | Surface density ( |
---|---|---|
100 ± 2.5 | 160 ± 5.41 | 53.7 ± 3.8 |
70 ± 2.5 | 110 ± 3.45 | 36.9 ± 3.0 |
35 ± 2.5 | 48 ± 7.4 | 16.1 ± 3.3 |
Figure
Scanning Electron Microscopy of the deposited film with 100 mg evaporated over the glass substrate.
Figure
Calibration curve for net area of K
As can be seen in Figure
Analysis of variance or ANOVA has been carried out to verify the quality of the adjustment of the model used in the calibration curve. The result is presented in Table
Analysis of adjustment variance of the model presented in Figure
Model | Source of variation | Quadratic sum | Degrees of liberty | Quadratic average |
|
---|---|---|---|---|---|
|
Regression | 0.70939 | 1 | 0.70939 | 0.98 |
Residues | 0.00102 | 6 | 0.00204 | ||
Total | 0.71041 | 7 |
Although
Three samples with different mass were produced with the same methodology used in the preparation of the calibration samples. Each sample had the size of 7.5 cm by 2.5 cm and the analyses were done in three different points. The net area of Ca-K
Thickness of unknown samples of the vanadium films calculated.
Mass | Net area | Thickness | Superficial density |
---|---|---|---|
(mg) | Ca-K |
(nm) | ( |
45 ± 2.5 | 9783 ± 108 | 62 ± 3.3 | 20.8 ± 0.2 |
55 ± 2.5 | 9512 ± 167 | 75.6 ± 4.1 | 25.4 ± 0.4 |
130 ± 2.5 | 8132 ± 93 | 185.7± 6.0 | 62.4 ± 0.9 |
Figure
Vanadium thickness on the glass substrate calculated using a calibration equation.
A typical spectrum obtained with the Portable X-Rays Fluorescence system for the sample of thickness
PXRF spectrum of vanadium layer on a glass substrate for the thickness of 185.7 nm. Measurement time 300 s.
In this spectrum we can clearly see the presence of other elements as Si, Ar, Ag, W, and Au. The presence of W may be attributed to a contamination that occurred in the sample preparation process at issue [
Thickness determined by PXRF for all the samples of V2O5 versus that determined by the deposited mass.
A variance analysis or ANOVA has been used to verify the adjustment quality for all the samples. The result is presented in Table
Adjustment of variance analysis of the points presented in Figure
Source of variation | Quadratic sum | Degrees of liberty | Quadratic average |
|
---|---|---|---|---|
Regression | 7202.264 | 4 | 7202.264 | 1 |
Residues | 0.01599 | 6 | 0.004 | |
Total | 7202.27999 | 11 |
Figure
Even though there is a relationship between the intensity attenuated by the film thickness and a theoretical exponential relationship, one can perform a linear approximation for the region to be examined as it has an extremely thin thickness range.
In this scientific work, thickness measurement of nanometric thin films made by a Portable X-Ray Fluorescence technique based on the attenuation of the radiation field is proposed. Although the methodology is simple, the use of calibration curves for unknown samples led to consistent results within the expected range for the thickness. In fact, the main issue which can eventually make interpretation of data difficult is the correct determination of the calibration curve using reliable standards. Therefore, this empirical method is fast and accurate for measuring thicknesses of nanometric films. In addition, it provides a simple and convenient method for thickness monitoring.
The authors declare that there is no conflict of interests regarding the publication of this paper.
Fabio Lopes acknowledges CAPES for eight months of Ph.D. grant at the University of Sassari. Luís Henrique Cardozo Amorin and Larissa da Silva Martins acknowledge CAPES for M.S. grant.