Chapter 5. Summary and Recommendation of Future Works
5.2. Recommendation of Future Works
Our study indicates that deposition parameters might influence the plasma characteristics during sputtering process and general physical properties in ZnO thin films. Moreover, investigation and discussion with different deposition parameters are an important for understanding the relationship between the plasma and physical properties. We suspect that there several critical deposition parameters influence the
above phenomena, including various working pressure, substrate temperature, distance between target and substrate, gas flow rate (gas flux), substrate rotation, etc. Based on the above parameters, we posit that the effect of various distances between target and substrate is the most important factor for the plasma situation.
The current measurement methods of material properties are not enough to interpret all of phenomena of material properties. For instance, although we obtained the hydrophobic characteristics of the ZnO thin film coating on the glass substrate, we still do not understand what mechanism causes it to produce high hydrophobic characteristics. However, we should acquire several measuring instruments to help us understand and interpret the phenomena of material properties and their relationship.
In the future work, we will need to use several measuring instruments, as following:
(1) scanning electron microscopy (SEM) or transmission electron microscopy (TEM), to photograph the morphology of the microstructure of ZnO thin film;
(2) atomic force microscopy (AFM), to record the surface roughness of ZnO thin film;
(3) fourier transform infra-red spectroscopy (FTIR), to determine the surface bonding of the ZnO thin film;
(4) photoluminescence (PL) measurement, to confirm the luminescence characteristics of ZnO thin film;
(5) stress measurement, to obtain the stress of ZnO thin film;
(6) nanoindentor, to observe the hardness of ZnO thin film;
(7) 4-point probe and Hall measurement, to measure the electrical properties;
(8) other mechanical apparatus, to take the mechanical properties.
Appendices
Appendix A. Langmuir Probe Analysis
The Langmuir probe is a powerful plasma diagnostic tool which is capable of determining the fundamental characteristics of plasma, such as the ion number density, electron number density, electron temperature, floating potential, plasma potential and electron energy distribution function (EEDF). The simplified schematic of a plasma diagnostic technique and a typical I-V curve are shown in Fig. 2.5 and Fig. 2.6, respectively. The complete process of measuring the I-V curve, analysing and displaying the results were described in following section.
For measuring the I-V curve, we inserted a small cylindrical probe into the plasma and applied a sweep voltage to the probe while measuring the resulting current drawn from the plasma. For example, the raw I-V curve was measured as illustrated in Fig. A.1.
According to this data, we could analyze and understand the variance of the plasma during the sputtering process. In the next section, we will describe the step-by-step analysis of the I-V curve by ESPsoft software (Hiden analytical software).
First, the floating potential is the easiest parameter to determine after we obtained the I-V curve. It is defined as the voltage at which the probe collects no current, specifically, when the net current is equal to zero, as shown in Fig. A.1. Then, we stated the square to the I-V curve and plotted the I2-V curve, as in Fig. A.2(a). We zoomed out
the ion saturation region to analyze the ion number density. The Orbital Motion Limited (OML) technique is considered briefly here. In short, a regression line is fitted to the I2-V curve in the ion saturation region, as present in Fig. A.2(b). According to the slope
of this line, the ion number density could be calculated by following equation:
15 1/ 2 1/ 2 (see Fig. A.3). Consequently, we could draw the electron I-V curve when the square root of the fitted line was added to the original I-V curve. This means that Fig. A.1 plus Fig.
A.3 equals Fig. A.4. This Ie-V curve neglects the ion current component and shifts the
current in the ion saturation region to zero.
Finally, the natural logarithm of the Ie-V curve is plotted from the above electron current characteristic, as revealed in Fig. A.5. The electron temperature, plasma (space) potential and electron number density were determined in this stage. The electron temperature is found when we choose a fitted line in the transition region, and is calculated by following equation:
1 Te
Slope
= − (A.2)
Then, another fitted line was plotted in the electron saturation region. Therefore, the intersection of the fitted electron saturation and transition lines is called plasma
potential. After we knew the electron temperature and plasma potential, the electron number density was acquired by following a simplification equation:
13 ,
The EEDF is also analyzed by ESPsoft software. However, the main method is the Druyvestyn method, which is calculated by the second derivative of the Ie-V curve.
Figure A.6 and Fig. A.7 exhibit the first and second derivative of the Ie-V curve,
respectively. The plasma potential was found at the maximum value of the 1st derivative or the zero of 2nd derivative, respectively. By observing the plasma potential, the EEDF
is determined as following:
( )
2 Eventually, the EEDF is plotted as shown in Fig. A.8.References
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