This study demonstrates a new approach for evaluating the properties of indium tin oxide (ITO) conducting glass and identifying defects using optical coherence tomography (OCT). A swept-source OCT system was implemented to scan the ITO conducting glass to enable two-dimensional or three-dimensional imaging. With OCT scanning, the defects can be clearly identified at various depths. Several parameters in addition to morphological information can be estimated simultaneously, including the thickness of the glass substrate, the refractive index, reflection coefficient, and transmission coefficient, all of which can be used to evaluate the quality of ITO conducting glass. This study developed a modified method for evaluating the refractive index of glass substrates without having to perform multiple scans as well as a segmentation algorithm to separate the interfaces. The results show the potential of OCT as an imaging tool for the inspection of defects in ITO conducting glass.
© 2011 OSA
Indium tin oxide (ITO) is one of the most common conducting oxides used on glass substrates due to the electrical conductivity and optical transparency. Indium tin oxide conducting glass has been widely used for industrial products such as display technologies, circuit substrates, and antennas for mobile communication [1–3]. Especially for display technologies including liquid crystal display (LCD), touch panel, organic light emitting diode (OLED), the qualities of ITO conducting glass are related to the light transmission efficiency of devices. Defects in ITO glass fundamentally influence its physical properties, such as electrical conductivity, optical transparency, and the spreading length of injected carriers, all of which determine the output performance of the display device. Therefore, the inspection of ITO conducting glass in manufacturing processes is highly critical. Many approaches have been demonstrated to inspect the defects of ITO conducting glass or industrial products [4,5]. Most of them have been based on the machine vision technique. With the high-resolution CCD and complicated software algorithm, the defects can be identified automatically. However, these approaches can only provide surface images without depth information. In addition, several parameters of ITO conducting glass are essential for evaluating quality, including substrate thickness, transmittance, and reflectivity. However, based on the machine vision technique, additional instruments are requested to obtain those parameters.
Optical coherence tomography (OCT) represents a technique for using low-coherence interferometers . The interfered signals of a Michelson interferometer are directly related to the intensity distribution of backscattered light that reflects the in-depth structure of a sample. With a low-coherence (broadband) light source, either fluorescence-based or from a laser source, the allowed interference range within the sample is small such that the imaging resolution in the depth direction is high. Recently, the development of the spectral-domain OCT (SD-OCT) [7–10] or swept-source OCT (SS-OCT) [11–13] has greatly improved the imaging speed and the operation sensitivity [14,15]. The imaging speed of SD-OCT/SS-OCT is higher than that of time domain OCT because the mechanical depth scanning (A-scan) is unnecessary. Based on high-speed scanning ability, it allows for animal and clinical studies with greatly reduced motion artifacts. In the last decade, OCT has been widely used for biomedical applications due to the advantages of non-invasive, high speed, 3D imaging in fields such as dermatology, oral mucosa, ophthalmology, and cardiology [16–20]. Aside from the biomedical applications, OCT is also used for art studies to authenticate ancient objects [21,22]. However, few efforts have been exerted on industrial products using OCT until now.
In this study, we propose a new approach for inspecting ITO conducting glass and evaluating the properties, which are related to output performance of the display device such as LCDs, OLEDs, and light emitting diode (LED) displays. Here, a method for estimating the refractive index of glass substrates is also demonstrated. According to the estimated refractive index, the thickness of the glass substrate, transmission coefficient, and reflection coefficient can be obtained as well. In this paper, the system and OCT scanning results of ITO conducting glass are described in Section 2. Based on OCT scanning results, the distribution of defect is discussed. In Section 3 of this paper, the method to estimate the refractive index is discussed and a segmentation algorithm is developed to separate the different interfaces. Then, in Section 4, the thickness of the glass substrate, transmission coefficient, and reflection coefficient are also obtained to evaluate the quality of ITO conducting glass. Finally, conclusions are drawn in Section 5.
2. System setup and OCT scan
Figure 1 shows the schematic diagram of the OCT system. A frequency-sweeping laser centered at 1.3 μm with a FWHM of 100 nm was operated at the sweeping rate of 30 kHz. The frequency-sweeping laser can provide 6 mW of output power and was connected to a Mach-Zehnder interferometer consisting of two couplers and two circulators. In the reference arm, a neutral density (ND) filter was inserted to maximize system sensitivity. In the sample arm, a pair of scanning galvanometers were implemented to provide the optical beam translation in X and Y dimensions. The interference fringe signal was then detected by a balanced photodetector (PDB150C, Thorlabs) and sampled by a high-speed digitizer (PXI-5122, National Instrument). With the sweeping rate of 30 kHz, the system could achieve a frame rate of 30 frames/s, each frame consisting of 1000 A-mode scans. In the three-dimensional (3D) data acquisition, the whole data could be acquired in 2 sec, each consisting of 600 x 300 x 200 voxels and spanning a range of 4 mm x 3 mm x 3 mm. System sensitivity can reach 105 dB. The system dispersion is compensated by a software compensation scheme . In our OCT system, the longitudinal and transverse resolutions are approximately 8 μm and 15 μm, respectively.
Before OCT scanning, a reflective mirror is placed on a precision stage, and a piece of defective ITO conducting glass is placed on the reflective mirror with perfect contact in the sample arm. 3D OCT imaging was performed on ITO conducting glass and Fig. 2 shows the OCT scanning results. Figure 2(a) represents a 3D OCT image with a volume size of 600 x 300 x 200 voxels. Figures 2(b) and 2(c) show the en-face images of the top and bottom surfaces extracted from the 3D OCT image, illustrating the defect distribution along the depth range, which is difficult to be obtained from the machine vision technique. Figure 2(d) represents the surface image of a normal ITO conducting glass examined by a microscope with a 10x objective lens. Compared with microscopic results, the same features can be found from the OCT images. Based on the OCT images, the defects in micron scale can be clearly identified.
3. Principle for the estimation of group refractive index
Although OCT images are useful for identifying defects in ITO conducting glass, optical properties are also important for quantitatively evaluating its quality. These optical properties include homogeneity, the thickness of the substrate, and transmission efficiency. The group refractive index can be used as an accurate indicator of the homogeneity of a glass substrate. Several methods have been proposed to calculate the refractive index of biological tissue or optical components [24–28]; however, it has been difficult to obtain the refractive index using the same measurements and it has been necessary to accurately control the depth of field in the sample arm. Kim et al.  demonstrated a novel concept to determine the refractive index by moving the positions of the mirror in the reference arm and the objective lens in the reference arm; however, the refractive index was difficult to obtain in a single measurement. In this study, we modified this concept to obtain a group refractive index simultaneously, without having to adjust for the difference in the optical path between the reference and sample arms.
Before scanning the sample with the OCT system, the depth calibration procedure had to be completed. A reflective mirror was used as the sample placed on a precision stage and was scanned with the OCT system. Based on the scanning result, the position of the reflective mirror indicated the optical path difference between the sample and reference arms and the position was also set as the reference plane for the scan of ITO conducting glass. The position of the mirror could then be altered by adjusting the precision stage. The range of change was available to estimate the depth resolution of our OCT system. In this study, the depth resolution was different from the longitudinal resolution, and was related to the sampling rate of high-speed digitizer. Based on the depth calibration, the size of each pixel through the range of depth was approximately 6.14 μm per pixel. Once the depth resolution had been determined, it remained applicable until the light source or the sampling rate of the digitizer was changed. In addition, when the position of reflective mirror was fixed and recorded, scanning the reflective mirror of the sample arm again was unnecessary unless the precision stage was adjusted. Therefore, we only needed to ensure the position of the reflective mirror was optimal when we turned on the OCT system. After recording the initial position of the reflective mirror and calibrating the depth resolution, the ITO conducting glass was placed on the reflective mirror with perfect contact. Figures 3(a) and (b) show the two-dimensional (2D) OCT images obtained from the reflective mirror, and ITO conducting glass placed on the same mirror, respectively. Figure 3(c) represents the A-mode scan profiles indicated by red dash lines in Figs. 3(a) and 3(b), respectively. The optical path difference in a single path, OPDG, caused by the ITO conducting glass can be expressed asEq. (1) and Eq. (2), n can be determined from the measured OPDG and OSM. As shown in Fig. 3(c), the positions of three peaks are indicated as 1160, 1836, and 2186 μm, which correspond to OPDG and OSM equal to 1026 and 350 μm, respectively. By substituting OPDG and OSM into Eq. (1) and Eq. (2), n is approximately 1.518. As a result, the thickness of ITO film is much thinner than that of a glass substrate; therefore, the refractive index and thickness of ITO film can be ignored here. To verify the accuracy of the estimated refractive index, a BK7 glass was used as the sample and the estimated refractive average of over 100 A-mode scans was approximately 1.5027 ± 0.0065, which is nearly the measured result equal to 1.503 in Ref .
4. Segmentation algorithm and properties evaluation
In this study, the glass substrate was placed to face the incident light and on the other side of ITO conducting glass, the ITO film directly contacted the reflective mirror. To estimate accurately the refractive index of glass substrate, a segmentation algorithm was developed to discriminate the different interfaces including the first plane of glass substrate, secondary plane of glass substrate, and the reflective mirror. Figure 4 shows the flow diagram of the data process. At the beginning of the flow diagram, the two peaks can be found in each A-scan profile of ITO conducting glass placed on the reflective mirror, corresponding to the first plane of the glass substrate and secondary plane of the glass substrate, respectively. Conversely, the maximum peak of each A-scan profile obtained from the reflective mirror represents the initial position of reflective mirror. Hence, OPDG can be acquired to calculate the difference between the first and second peaks. OSM can be obtained by calculating the difference between the second peak of A-scan profiles of ITO conducting glass placed on the reflective mirror and the maximum peak of A-mode scan profiles of the reflective mirror, as shown in Fig. 4. According to Eq. (1) and Eq. (2), several parameters can be evaluated simultaneously including the refractive index, thickness of glass substrate, reflection coefficient, and transmission coefficient.
Based on the segmentation algorithm, the refractive index distribution of Fig. 2(a) could be obtained, as shown in Fig. 5(a) . The mean value of refractive index is demonstrated in Fig. 5(a) to give approximately 1.5312 and the standard deviation of the refractive index in Fig. 5(a) is also evaluated to give 0.0559. From the result, one can see that the smaller values of refractive indices in Fig. 5(a) correspond to the locations of defects in Fig. 2(a). The larger standard deviation compared with that of BK7 material results from the existence of defect. Hence, the evaluation of the refractive index of ITO conducting glass can also be useful for inspecting the quality of ITO conducting glass. With the estimated refractive indices, the thickness distribution of glass substrate can also be determined from Eq. (1). Figure 5(b) represents the thickness distribution of the glass substrate evaluated from the same 3D OCT data of Fig. 2(a). The mean value of thickness is demonstrated in Fig. 5(b) to give approximately 658.1 μm, and the standard deviation of the thickness in Fig. 5(b) is also evaluated to give 5.2 μm. Due to the uniformity of transmission efficiency being associated with the uniformity of thickness, the thickness can also be used as an accurate indicator for evaluating the quality. In addition, the reflection and transmission coefficients of glass substrate can be acquired using Eq. (3).Figs. 6(a) and 6(b), respectively. Here, the mean values of Figs. 6(a) and 6(b) are also estimated to give 0.9559 and 0.0441, respectively, and each of the standard deviations in Figs. 6(a) and 6(b) is equal to 0.0054. Here, one can see that the reflection and transmission coefficients can also be accurate indicators for property evaluation of ITO conducting glass.
In this paper, we propose a modified method to evaluate the refractive index. This method is only suitable to be used for the transparent samples for acquiring the position of the reflective mirror; therefore, evaluating the refractive index of the opaque samples is difficult. In this paper, we show only the results of the ITO film contacting the reflective mirror. However, the segmentation algorithm and data process for property evaluation can also be used when the ITO film is faced toward the incident light.
To summarize, we demonstrate a new approach for defect inspection and property evaluation of ITO conducting glass using OCT. The machine vision technique is the most common method used for defect detection for ITO conducting glass, but can only provide surface images without depth information causing difficulty identifying the defects below the glass surface. Compared with the machine vision technique, OCT can reconstruct the three-dimensional microstructure of a sample at a depth range of 2-3 mm. Using OCT scanning, the defects can be clearly identified at various depths. Several parameters in addition to morphological information can be estimated simultaneously, including the thickness of the glass substrate, the refractive index, reflection coefficient, and transmission coefficient, all of which can be used to evaluate the quality of ITO conducting glass. In this study, a modified method is proposed to evaluate the refractive index of glass substrates without multiple scans; and a segmentation algorithm to separate the different interfaces is also developed. From the results, one can see that OCT could be a potential imaging tool for defect inspection of ITO conducting glass or other industrial products such as flexible printed circuit boards, blue-ray discs, and OLED displays.
This research was supported by National Science Council, the Republic of China, under the grants of NSC 99–2221–E–182–044 and NSC–98–2112–M–003–001–MY2, and also by Chang Gung University, the Republic of China, under the grant of UERPD290011.
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