Experimental Instrumentation

There are four main kinds of experimental instrumentation in the following section, including plasma diagnostics, as well as structural, optical, and chemical characteristics and techniques, respectively.

3.2.1. Plasma Diagnostics

In the following section, we will be describing the operating limits, types, and measurement methods of the Langmuir probe system.

3.2.1.1. Langmuir Probe System

The configuration and schematic of a standard ESP analysis system with motorized Z-motion drive (Langmuir probe system) are displayed in Fig. 3.13 and Fig. 3.14, respectively. A complete Langmuir probe system comprises an ESPION Probe Unit (EPIU), a linear motion driver, a gas-cooled RF compensated electrostatic plasma probe, and all the required interconnection cabling. This Langmuir probe system is a

cylindrical type made by Hiden Analytical Inc. (Model: Hiden ESPion). The configuration of a cylindrical probe includes an RF-compensation electrode and probe tip (see Fig. 3.15). All of these components of the cylindrical probe are made from insulator material except for the probe tip. The purpose of the insulator material is to decrease perturbation of the plasma. Figure 3.15(b) shows the connector pin, metal ferrule, ceramic tube and tip wire. The probe tip (whose dimensions appear in Fig.

3.15(c)) also plays a key role in plasma diagnostics by reducing perturbation. It is made

of tungsten, and is 10 mm in length and 0.15 mm in diameter, with an estimated effective collection area of about 4.7 mm². The probe tip was placed in the plasma bulk region, 2 cm from the substrate along the center line (see Fig. 3.2). This standard Langmuir probe system has a probe current and tip voltage range of 1 mA to 1 A and of -200 V to 100 V, respectively. It also suitable to operate with both DC and RF plasmas when using various compensation electrodes. The maximum allowable temperatures at the probe and at the mounting flange are 250℃ and 70℃, respectively. If the plasma diagnostics are measured at high temperature, a cooling gas inlet is provided to force the probe compensation components to cool. Hiden Analytical Inc. suggests that the cooling gas (such as air, nitrogen, argon, etc.) may be used at a recommended pressure between 150 torr and 525 torr (3 psi to 10 psi). An automatic motor-driven Z-motion driver is an available option for the Langmuir probe system (see Fig. 3.17). A computer-controlled

stepped motor, with maximum movement of 300mm, powers the automatic Z-motion driver. The stepped motor is controlled by the PC via the EPIU and the Linear Motion Driver as shown in Fig. 3.14. A lead screw drives the probe into the chamber; vacuum sealing is provided by the flexible bellows unit.

3.2.2. Structural Characterization Techniques

In the following section, we will describe the operating limits, types, and measurement methods of surface profile and x-ray diffraction, respectively.

3.2.2.1. Surface Profilometer (α-step)

In Fig. 3.18, film thickness was measured by a surface profilometer (Model 3030, Sloan Dektak Inc.). This is a simple, rapid, and convenient measurement method for obtaining film thickness or surface roughness degree. The measurement range is from 50 Å to 1310 KÅ with a maximum of vertical and horizontal resolution of 1 Å and 0.025 µm (250 Å), respectively. The scan length range is from 50 µm to 50 mm.

3.2.2.2. X-Ray Diffractometry (XRD)

Crystal structure and the degree of preferred orientation of ZnO thin films was determined by X-ray diffraction (XRD) technique (Model: PANalytical X'Pert Pro (MRD), Philips X’Pert Inc.) as shown in Fig. 3.19. Figure 3.20 shows the goniometer, an instrument that consists of an x-ray tube, primary optics, MRD candle, secondary optics, and a detector. The x-ray source used in the X’Pert Pro MRD is a ceramic

filament tube with a copper (Cu) target (anode) using CuKα radiation (λ=1.5418 Å).

The detector converts the received x-ray photons into electrical pulses that are individually counted [33]. The unit of counts—counts per second (cps)—is employed for measuring the intensity scattered by the sample. The angular resolution for the instrument is around 0.001˚. This instrument was operated at 45 kV and 40 mA. In the present study, two types of XRD scans were performed on different samples: the Gonio scan and GIXRD scan. The Gonio scan is the most commonly used technique for measuring the Bragg reflection of a thin film, and is also called the θ-2θ scan. The Gonio scan analyzes position, shape, intensity, etc., in order to determine the microstructure information of material. In the GIXRD (grazing incidence x-ray diffraction) scan, the primary beam enters the sample at very small angles of incidence.

It is also called the ω-2θ scan. The angle between the incidence beam and the sample surface is very small and amounts to only a few degrees or even less (in general, 1˚~6˚).

The x-ray path travels a small entrance angle is increased the significantly and the structural information. The simple diagram of these two measured techniques is presented in Fig. 3.21.

3.2.3. Chemical Characterization Techniques

In the following section, we will describe the system and operating limits of X-ray Photoelectron Spectroscopy (XPS).

3.2.3.1. X-ray Photoelectron Spectroscopy (XPS)

The composition and chemical state of ZnO thin film was studied with X-ray Photoelectron Spectroscopy (XPS) (Model: VG Scientific Microlab 310F). XPS, also called Electron Spectroscopy for Chemical Analysis (ESCA), is the most widely used surface analysis technique because of its relative simplicity in use and data interpretation. Its element detection range spans from Li to U (atomic number: 3~92).

This instrument is based on vacuum systems designed to operate in the ultra-high vacuum (UHV) range of ~10^-9 torr [156]. The UHV environment is necessary because of the surface sensitivity of the techniques themselves and the contamination reduction of sample surfaces by absorbed residual gas molecules [157-158]. The maximum specimen size limit is 1 cm × 1 cm and the thickness is lower than 0.5 cm. X-rays are generated by bombarding an anode material with high energy electrons from a heated filament [158]. In this instrument, the X-ray source was made by twin anode X-rays, MgKα (1253.6 eV) and AlKα (1486.6 eV). (Aluminum target was used in this study).

The electron gun resolution is 15 nm at 25 keV using the Schottky Field Emission Source. In addition, the electron energy analyser measures the energy distribution of

electrons emitted from the specimen. Here it uses a Concentric Hemispherical Analyzer (CHA) mode and its resolution is 0.02% ~2%. However, XPS is suitable for analysis of surface chemical characteristics because it can simultaneously indentify the chemical state of various elements. Eventually, we employed this instrument to detect the composition and chemical state of ZnO thin film.

3.2.4. Optical Characterization Techniques

In the following section, we will describe the operating limits, types, and measurement methods of ultraviolet-visible (UV-VIS) spectroscopy.

3.2.4.1. Ultraviolet-Visible Spectrophotometer (UV-VIS Spectrophotometer)

Figure 3.22 presents a measurement of transmittance and absorption of the ZnO thin film, carried out using a UV-VIS spectrophotometer (Shimadzu Inc., Model UV-2501PC). The system consists of an intelligent photometer unit, IBM PC/AT compatible series of personal computers, and the UVPC Personal Spectroscopy Software package. Data are acquired through three basic modes: wavelength scanning, quantitative (Single Wavelength), and time scanning, with the software controlling all acquisition parameters and storage formats. In the light source compartment, a halogen lamp is built in for visible region, and a deuterium lamp for UV region. The scanning region of UV-VIS spectrophotometer is from 190 nm to 1100 nm, but the effective scanning region is from 190 nm to 900 nm. The light source switching wavelength is

selectable from 282~393 nm. However, the spectra of transmittance and absorption are obtained by the UV-VIS spectrophotometer.

3.2.5. Hydrophobic/Hydrophilic Characterization Techniques

In the following section, we will describe the operating limits, types, and measurement methods of the contact angle system.

3.2.5.1. Contact Angle System

In Fig. 3.23, the surface characteristics of the contact angle were recorded by a contact angle system with a universal surface tester (Model GH-100, KRŰSS Inc.), which can measure contact angle and surface energy. The system consists of an automatic sample positioning system for process control, PC-controlled motor-driven zoom and focus, 2/3” CCD camera, illumination unit and process measuring head. Four kinds of liquids can be used for analyzing the surface energy of materials. Water (H2O), Diiodo Methane, and Ethylene Glycol are always provided in the first three needles and the last one is a standby for special cases. The system has three different measurement modes, including static mode, advance mode, and recede mode. The measuring range of the contact angle system is from 2˚ to 175˚ and the resolution is 0.1˚. However, we are solely interested in the surface characteristic of contact angle degree in present study.