3.3.1 Scanning Electron Microscopy (SEM)
SEM stands for scanning electron microscope. The SEM is a
microscope that substitute electron for light to form an image. Scanning electron microscopes have developed new areas of study in the medical and physical science communities since their development in the 1950.
SEM uses a focused beam of high energy electrons to generate a variety of signals at the surface of solid samples. The signals that derive from electron and sample interactions reveal information about the sample including external morphology, chemical composition, and crystalline structure and orientation of materials making up the sample. The researchers can inspect a much larger variety of samples by SEM.
Accelerated electrons carry significant amounts of kinetic energy in the SEM. When the incident electrons are decelerated in the solid sample, this energy is dissipated as a variety of signals produced by electron and sample interactions. These signals include secondary electrons, backscattered electrons, diffracted backscattered electrons, photons and heat. Secondary electron and backscattered electrons are commonly used for imaging samples. Secondary electrons are most valuable for showing morphology and topography on samples and backscattered electrons are most valuable for illustrating contrasts in composition in multiphase sample. X-ray generation is produced by inelastic collisions of the
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incident electrons with electrons in discrete orbital of atoms in the sample.
X-rays are fixed wavelength as the excited electrons return to lower energy states. Thus, characteristic X-rays are produced for each element in a mineral that is excited by the electron beam. SEM analysis is considered to be non-destructive; that is, X-rays generated by electron interactions do not lead to volume loss of the sample, so it is possible to analyze the same materials repeatedly.
The SEM has many advantages over traditional microscopes. We describe it following:
(1) It has large depth of field, which allows more of a specimen to be in focus at one time.
(2) SEM has much higher resolution so closely spaced specimens can be magnified much higher levels.
(3) Researcher has more control in the degree of magnification, because of the SEM uses electromagnets rather than lenses.
3.3.2 Transmission electron microscopy (TEM)
Transmission electron microscopy (TEM) is a microscopy technique whereby a beam of electrons is transmitted through an ultra-thin specimen, interaction with the specimen as it passes through. The interaction of the electrons transmitted through the specimen form an image that is magnified and focused onto an imaging device, such as a fluorescent screen, on a layer of photographic film, or to be detected by a sensor such as a charge coupled device (CCD) camera.
Owing to the small de Broglie wavelength of electrons, TEMs are
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capable of imaging at a significantly higher resolution than light microscopes. Therefore, the instrument’s user to examine fine detail even as small as a single column of atoms, which is tens of thousands times smaller than the smallest resolvable object in a light microscope. TEM is an important analysis method in the physical and biological sciences fields that application in cancer research, virology, materials science as well as pollution, nanotechnology, and semiconductor research.
At smaller magnifications TEM image contrast is due to absorption of electrons in the material, due to the thickness and composition of the material. At higher magnifications complex wave interactions modulate the intensity of the image, requiring expert analysis of observed images.
Alternate modes of use allow for the TEM to observe modulations in chemical identity, crystal orientation, electronic structure and sample induced electron phase shift as well as the regular absorption based imaging.
3.3.3 Atomic Force Microscopy (AFM)
Atomic force microscopy is a manner of measuring surface morphology on a scale from angstroms to 100 microns. The technique involves imaging a sample through the use of a probe or tip, with a radius of 20 nm. The tip is held several nanometers above the surface using a feedback mechanism that measures surface tip interactions. Variations in tip height are recorded while the tip is scanned repeatedly across the sample, producing a topographic image of the surface.
In addition to basic AFM, the instrument in the Microscopy Suite is
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capable of producing images in a number of other modes, including tapping, magnetic force, electrical force and pulsed force. In tapping mode, the tip is oscillated above the sample surface, and data may be collected from interactions with surface morphology, stiffness and adhesion. This result in an expanded number of image contrast methods compared to basic AFM. Magnetic force mode imaging utilizes a magnetic tip to enable the visualization of magnetic domains on the sample. In electrical force mode imaging a charged tip is used to locate and record variations in surface charge. In pulsed force mode, the sample is oscillated beneath the tip, and a series of pseudo force distance curves are generated. This permits the separation of sample topography, stiffness, and adhesion values, producing three independent images, or three individual sets of data, simultaneously.
3.3.4 X-Ray Diffraction (XRD)
X-ray diffraction (XRD) is one of the most important techniques for qualitative and quantitative analysis of crystalline compounds. The XRD technique provides information includes types and nature of crystalline phase present structural makeup of phase, degree of crystallinity, amount of amorphous content which microstrain and size and orientation of crystallites.
When a sample is irradiated with a parallel beam of monochromatic X-ray, the atomic lattice of the sample acts as a three dimensional diffraction grating causing the X-ray beam to be diffracted to specific angles. The diffraction pattern that includes position (angles) and
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intensities of the diffracted beam that provides several types of information about the sample which are discussed below:
Angles are used to calculate the interplanar atomic spacing (d-spacing). Because every crystalline material will give a characteristic diffraction pattern and can act as a unique “fingerprint”, the position “d”
and intensity “I” information are used to identify the type of material by comparing them with patterns for over 80,000 data entries in the International Powder Diffraction File (PDF) database. Hence, identification of any crystalline compounds, even in a complex sample by XRD.
The position “d” of diffracted peaks also provides information about how the atoms are arranged within the crystalline compound (unit cell size or lattice parameter). The intensity information is used to assess the type and nature of atoms. Determination of lattice parameter helps understand extent of solid solution (complete or partial substitution of one element for another, as in some alloys) in a sample.
Width of the diffracted peaks is used to determine crystallite size and microstrain in the sample. The “d” and “I” from a phase can also be used to quantitatively estimate the amount of that phase in a multicomponent mixture. As mentioned earlier, XRD can be used not only for qualitative identification but also for quantitative estimation of various crystalline phases. This is one of the important advantages of the X-ray diffraction technique.
3.3.5 Ultraviolet-Visible Spectroscopy
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Ultraviolet-visible spectroscopy (UV-Vis) refers to absorption, transmission or reflectance spectroscopy in the ultraviolet-visible spectral region. This means it uses light in the visible and adjacent (near-UV and near-infrared (NIR)) ranges. The absorption, transmission or reflectance in the visible range directly affects the perceived color of the chemicals involved. In this region of the electromagnetic spectrum, molecules undergo electronic transitions. This technique is complementary to fluorescence spectroscopy, in that fluorescence deals with transitions from the excited state to the ground state, while absorption measures transitions from the ground state to the excited state [72].
Transmission spectroscopy is highly interrelated to absorption spectroscopy. This technique can be used for solid, liquid, and gas sampling. Here, light is passed through the sample and compared to light that has not. The output depends on the path -length or sample thickness, the absorption coefficient of the sample, the reflectivity of the sample, the angle of incidence, the polarization of the incident radiation, and, for particulate matter, on particle size and orientation. In the Beer-Lambert Law, the equation IT/TO is called transmittance. This form of spectroscopy has a setup similar to the one used for absorption.
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Table 3-1 APPJ experimental parameters
Parameter value
Substrate temperature (°C) 200
Gap distance (mm) 5
Scan times 10
Carrier gas flow rate (sccm) 30
Main gas flow rate (SLM) 35
Carrier gas Nitrogen
Main gas Nitrogen
Nozzle speed (mm/s) 20
Ultrasonic frequency (MHz) 2.45
Figure 3-2 Schematic of CVD reaction steps [71]
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Figure 3-3 Schematic of APPJ system
Figure 3-4 Scanning path
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Figure 3-5 APPJ system of ITRI.
Figure 3-6 Patterns on mask
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(a) N+ Silicon wafer after RCA Clean
(b) Gate insulator deposited
(c) IGZO thin film deposited by APPJ
(d) Define active layer region
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(e) Spin coating photo resist on Si wafer
(f) Defined electrodes region
(g) Aluminum deposited by E-Gun
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(h) Lift off P.R and Al on it
(i) Electrons stayed on active layer
Figure 3-7 Schematic of experimental procedures. Pictures (a) to (i) show the lift-off process.
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Figure 3-8 IGZO TFTs structure on silicon substrate
Figure 3-9 Schematic illustration of SEM instrument.
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Figure 3-10 Schematic illustration of TEM instrument.
Figure 3-11 Schematic illustration of AFM instrument.
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Table 3-2 Characterization analysis equipments
Instrument Company and type
I-V Agilent 4156C
SEM Hitachi S-4700I
TEM JEOL JEM-2010F
AFM Veeco Dimension 3100
XRD PANalytical X'Pert pro
XPS PerkinElmer PHI1600
UV-VIS-MIR VASCO V-570
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Chapter 4
Results and Discussions
In this chapter, we will discuss and explain the characterizations of IGZO thin films and the effect of annealing on IGZO films by SEM, AFM, XRD, XPS, TEM and UV-VIS-MIR. Next, we will compare different annealing temperatures of TFTs electrical characteristics. Finally, to improve the TFT electrical characteristics and increase the applications of TFT, we substitute the insulator of SiO2 with HfO2 and Al2O3. We will compare the different high-k material for better gate dielectric and electrical characteristics. Consequently we will discuss the following conditions:
1. The annealing temperature :
(1) 200°C (2) 300°C (3) 400°C (4) 500°C
2. The annealing ambiance : (1) N2 (2) O2
3. The gate insulator :
(1) SiO2 (2) HfO2 (3) Al2O3
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