Chapter 3 Experimental
3.3 Instrumentation
The stresses of the thin films as a function of temperature were obtained by measuring the radius of curvature of the films on silicon strips using a home-built bending beam system.
The measurement bases on the change of the laser light reflection on the sample surface when sample bends due to the stress induced in the coated film. Figure 3.9 details the stress measurement principle. A laser beam from the He-Ne laser source was split into two beams by the beam spitting cube and mirror system before reaching the surface of sample, which is placed inside the vacuum chamber. The sample is required to be mirror-like in order to reflect the two laser beams as they arrive on the sample surface. A double-side polished wafer can serve as a good reflector for this purpose. In the measurement, the sample is heated inside a Cu heating chamber. The directions of two reflected laser beams changed with the change of temperature due to the sample bending. A position sensor system is used to detect the reflected laser beams position. The optical signal is amplified by an amplifier, and then changed into the analog signal, and finally transferred to a computer through data acquisition interface card (national Instruments) and Labview program. Through the integration of computer, data acquisition, and software, the measurement system can record the bending curvature of sample from the distance between two reflected laser spots as a function of thermal cycling as described latter. The stress of a thin coated film based on Stoney’s Eq.[59]
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Where, Es is the Young’s modulus of the substrate. ʋ s is the Poisson ratio of the substrate, ts and tf are the thickness of the substrate and the film, respectively. R is the radius of curvature of the film on the substrate and Ro is the radius of curvature of the bare substrate after the thin film is removed. The term of (1/R – 1/R0) is so-called the bending curvature of a sample. The Eq. (3.1) assumes that both film and substrate are isotropic and film is much thinner than substrate (tf < 0.06 ts)[59].
Figure 3.10 illustrates the principle for obtaining the bending curvature of a sample in the bending beam system. It is considered the sample before bending as a line along the x axis, and y axis is normal to the x axis. Under the lab coordinate, the deflection of the sample during the bending is a function of x: y = f(x). When the dy/dx is small, then the sample bending curvature is related to the second derivative [60]
1 R
1
Ro d2y
dx2 (3.2)
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Figure 3. 9 Schematic diagram of a bending beam system
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Figure 3. 10 Principle for determining the bending curvature of sample [60]
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In Figure 3.10, AO is the incident laser beam. O and O ’ are normal to the sample surface before and during thermal loading. TO is the tangent line of the curve at the laser incident point and OB and OC are reflected beams from the sample before and during thermal
loading. From the geometry, it is clear that:
2 2 (3.3) (3.4)
Thus
1
2 (3.5)
When the incident angle and the deformation of the package are small:
dy
dx x x0 tan 1
2 x
2 (3.6)
Where L is the optical path and x is the displacement of the reflected laser spot on a photon position-sensitive detector during thermal loading. By using two laser beams and two detectors, dy/dx can be measured at two different points, x1 and x2. When D = x2- x1 is relatively small compared with the radius of curvature of the line, the second derivative can be calculated by Eq. (3.7)
Combining Eq.s (3.6) and (3.7), the bending curvature is expressed as below:
(3.7)
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R 1
R0 ddx2y2 x2D 1 x2 S2D 0 S (3.8)
Where S is the distance between two laser spots on position sensor as the substrate is with film, S0 is the distance between two laser spots on position sensor as the substrate without film (See Figure 3.11).
Figure 3. 11 Spatial relationship among parameters in Eq. (3.8)
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In the present work, for the samples ITO/Si, the distances S0 were measured before the ITO film deposition. Then, the distances S were obtained after coating ITO on substrate.
However, for cases of polymer substrates, the S0 were also determined with Si wafer. The stress sample size is only (5 cm × 1cm) however the spin-coating of the polymers requires a square wafer. Therefore, the distances S were first determined after depositing ITO film on polymer (PI or PET)/Si substrate. Then, the film and polymer (PI or PET) were taken out the Si wafer. Subsequently, the S0 were verified. The distance between two incident laser beams, D, equals to 4 cm. The optical path, L, is 2 m long.
3.3.2 Scanning electron microscope
The morphology of ITO film was examined by a scanning electron microscope (SEM) HITACHI – S2500 JSM – 6700.
SEM is a type of electron microscope that uses the high energy beam to image the sample surface. The electron react with atoms in sample surface and products the information of surface sample topography[61]. In a typical SEM shown in Figure 3.12, an electron beam is thermionically emitted from an electron gun fitted with a tungsten filament cathode[62].
The electron beam, which typically has an energy ranging from 0.5 KeV to 40 KeV, is focused by one or two condenser lenses to a spot about 0.4 nm to 5 nm in diameter. The beam passes through pairs of scanning coils or pairs of deflector plates in the electron column, typically in the final lens, which deflect the beam in the x and y axes so that it scans in
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a raster fashion over a rectangular area of the sample surface.
Figure 3. 12 Schematic diagram of an SEM
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3.3.3 X-ray diffraction
X-ray diffraction (XRD) is a non-destructive tool for analyzing material properties such as crystallinity, the phase identification, and orientation.
XRD was utilized to analyze the crystallinity in this study. The samples were scanned from 20 degree to 60 degree by Siemens Diffractometer D8000 with small angle ~ 3 degree and step size ~ 0.02 degree. The Xray source is used in the system is Cu K (λ=1.5405Å).
During scanning period, X-ray beam of wavelength λ was irradiated to the sample at an angle , and the diffracted intensity at an angle 2 was recorded by a detector as illustrated in Figure
3.13.
Figure 3. 13 Definition of the angle of incidence and diffraction in an XRD experiment
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3.3.4 Hall Effect measurement
Hall Effect refers to potential difference (Hall voltage) on opposite sides of a thin sheet of conduction or semiconducting materials through, which an electric current of flowing, created by a magnetic field applied perpendicular to the Hall element.
From the Hall Effect measurements, the following electrical properties of material can be obtained
(1) The sheet resistance, from which the resistivity can be determined for a sample with a given thickness.
(2) The sheet carrier density of the majority carrier (the number of majority carriers per unit volume). The density of semiconductor (doping level) can be found for a sample with a given thickness.
(3) The mobility of the majority carrier.
(4) The doping type of material.
Taking measurements
(1) Current IAB is a positive DC current measured in amperes
(i) Injected into contact A
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(ii) Take out of contact B (2) Voltage VDC is DC voltage (V)
Measure between contacts C and D with no externally applied magnetic filed
(3) Sheet resistance Rs is measured in Ohms (Ω) by applied the Ohm’s law shown in Eq.
(3.9)
RAB,CD = VCD / IAB (3.9) Taking several reciprocal measurements and averaging to get more precise results.
In this study, the carrier concentration of ITO on PET and ITO on PI samples were obtained by employing a Van der Pauw technique Bio-Rad Micro-science HL 5500 PC. The voltage between contacts C and D, VCD, was 20 mV.
3.3.5 UV-visible spectrophotometer system
An analytical technique for the measurement of wavelength-dependent attenuation of ultraviolet, visible and near-infrared light and used in the detection, identification and quantification of atomic and molecular species. A UV/VIS spectrophotometry V670 (JASCO Corporation) was used to determine the absorption or transmission of UV/VIS light (180 to 900 nm) by a sample. It could also be used to measure concentrations of absorbing materials based on developed calibration curves of the material. Ultraviolet-visible (UV-vis) spectra of the polymer films were recorded on a HP G1103A spectrophotometer.
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