The display quality of touchscreen devices with on-screen fingerprint sensors is reduced by moiré patterns, interference phenomena caused by an overlap between the pixel pattern of the display, and the electrode pattern of the fingerprint sensor. A promising strategy for resolving this issue is to reduce the visibility of the moiré pattern, by including a filling layer with a transmittance similar to that of the electrodes, between the different patterns. We propose a moiré-free fingerprint sensor that uses an oxide-metal-oxide (IZO/Ag/IZO) multilayer as a highly transparent electrode. To verify the moiré reduction effect, we conducted a two-dimensional spectral analysis to calculate the spatial frequencies of the superimposed image of the display and the sensor patterns, and demonstrated experimentally that the proposed electrode greatly reduces the undesirable moiré phenomenon.
© 2018 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
Transparent conductive materials are an essential component of optoelectronic devices such as liquid-crystal displays (LCDs), touch panels, organic light emitting diodes (OLEDs), and solar cells. The high conductivity and transparency of indium tin oxide (ITO) has made it the standard choice as an electrode material for such applications [1,2]. However, the use of ITO is still greatly limited by the high cost of the raw material, poor mechanical flexibility due to its inherent brittleness, and the requirement for high-temperature deposition conditions . Moreover, ITO electrodes exhibit a trade-off between the sheet resistance and optical transmittance, characteristics which both need to be optimized in a transparent electrode [4–8], i.e., increasing the ITO thickness to reduce its sheet resistance has the disadvantage of reducing its transmittance. Materials, such as Al-doped ZnO (AZO), carbon nanotubes, metal nanowires, ultrathin metals, conductive polymers, and graphene, have thus been studied in an effort to overcome the disadvantages of ITO [9–17]. More recently, oxide-metal-oxide (OMO) electrodes have been developed, to lower the sheet resistance and improve the light transmittance of the transparent conductor [18,19]. A recent study reported an OMO electrode with a transmittance of 98% and a sheet resistance of less than 6 Ω sq−1 . This electrode consists of a thin Ag metal layer, which reduces the sheet resistance of the device, placed between a TiO2 undercoat layer and an AZO overcoat layer, which reduce the reflectance of the metal layer through destructive interference.
Bezel-less or fullscreen smartphones, where the size of the display screen and the overall usability of the device are enhanced, while retaining its portability, are increasingly prevalent. Because of the disappearance of physical buttons from the front of these devices, smartphone manufacturers have developed or adopted sensors, which accept fingerprint input on the display screen, to replace the home button. Transparent capacitive-type on-screen fingerprint sensors are an example of one such input device under development. This type of sensor requires transparent conductive materials such as ITO, OMO, and metal meshes, for use as electrodes. However, as these electrodes are not perfectly transparent, they inevitably interfere with the light emitted from the display, a process which can degrade the display quality. In particular, when the repetitive pattern of the display overlaps with the repetitive pattern of the sensor, a moiré pattern that can significantly deteriorate image visibility can be observed . As this moiré effect is a form of optical interference, its appearance can be avoided by using nonperiodic or randomly shaped patterns, or by changing the pitch of the two patterns, and/or their rotation angle [21,22]. The pitch of the electrodes in a capacitive touch sensor is much larger than the pitch of the smartphone’s pixel arrays (several millimeters vs. around 100 μm), and the creation of a moiré pattern can thus be easily avoided, without affecting the sensing capability, by rotating the sensor pattern and/or changing the shape of the pattern. However, with fingerprint sensors, the inter-electrode pattern period should be less than 100 μm, corresponding to a resolution of 250 pixels per inch (ppi), for this to be sufficient for fingerprint recognition. At the same time, this period cannot be much smaller than several tens of micrometers, for the electrodes to generate a large enough signal for sensing. Therefore, when a fingerprint sensor is assembled on a smartphone display, there is an overlap of patterns with similar periods, and avoiding or reducing the moiré phenomenon using simple rotations, or slight changes to the shape of a pattern, is difficult.
In this study, we present a transparent electrode that minimizes the moiré phenomenon. We analyzed the moiré phenomenon in the frequency domain to observe the effect of the OMO electrode in minimizing the interference between the fingerprint sensor and the display pattern. We also measured the electrical and optical properties of the proposed OMO electrodes, as well as the degree of moiré reduction, and discussed these results in relation to their potential use as on-screen fingerprint sensors.
2. Simulation and reduction of the moiré phenomenon
Because of the limit on the sensing distance, an on-screen fingerprint sensor needs to sit ‘on’ the display panel and be protected with an ultrathin (100–200 µm) cover glass, as shown in Fig. 1(a). This configuration makes the occurrence of the moiré phenomenon more likely, due to the interference between the periodic pixel structure of the display and the periodic fingerprint sensor pattern. Figures 1(b) and 1(c) show the unwanted moiré patterns generated when fingerprint sensors composed of typical diamond electrodes with a 70-µm pitch are assembled, without any rotation, on smartwatch and smartphone displays, respectively. In this image, the pixel pitch of the smart watch is approximately 91.5 µm, and the pixel pitch of the smartphone is approximately 44.5 µm. Moiré patterns that degrade the image quality of the display are clearly visible in these figures. It is therefore important to predict and analyze moiré patterns in order to avoid or minimize their appearance.
In order to predict the moiré phenomenon, we performed a two-dimensional (2D) spectral analysis using MATLAB, to calculate the spatial frequencies of the superimposed image of the display and the sensor pattern used in this work. The RGB image of the OLED pixel array was converted to grayscale in order to obtain Idisplay(x,y), the 2D intensity distribution, and multiplied element-wise with Tsensor(x,y), the 2D transparency distribution of the fingerprint sensor array, to simulate the superpositioning of the display pixel pattern and the sensor pattern in the image domain. Parts of the grayscale images of the OLED pixel array and the sensor pattern array are shown in Fig. 2(a). After multiplication, we conducted a discrete Fourier transform (DFT) on the N × N image intensity distribution function, f(x,y) = Idisplay(x,y)⋅Tsensor(x,y), to obtain F(u,v) as follows:
We also extracted the patterns of bright and dark areas of the overlapping 2D images, directly from the image domain, as a complementary approach to spectral analysis, for predicting the moiré pattern. We used the pattern extraction function from RaywizMOIRE (Insideoptics, Inc.), a commercial simulation software package, for this purpose. Although details regarding the operation of this function were withheld, it appears that the program simply converts an overlapped grayscale image and its inverse into binary images, and suggests the two binary images as expected moiré patterns. In most cases, one of the two binary images agrees well with the observation, as can be seen in the extracted pattern shown in Fig. 2(c), which looks very similar to the observed moiré pattern illustrated in Fig. 1(c).
With existing touch sensors, the pattern to be identified, when the pixel pattern of the display and the electrode pattern of the sensor overlap, is the same as if only the electrode pattern was considered, i.e., there is no moiré pattern, and only the large touch electrode pattern is visible. In general, the pattern visibility problem of an ITO-electrode touch sensor can be solved simply by using index matching with a low-reflection coating. With a fingerprint sensor, the period of the electrode pattern is similar to that of the pixel pattern of the display, and each of these patterns cannot be visually identified. However, when these two patterns overlap, a pattern with a new period appears in the visibility circle. This newly occurring moiré visibility problem is specific to devices where the period of the fingerprint sensor’s electrode is similar to the period of the display’s pixel pattern.
As mentioned above, when the newly formed moiré pattern is placed in the visibility circle, it can be observed visually, affecting the display quality of associated devices. Avoiding the generation of an optical interference pattern, by minimizing the difference in refractive index between the conducting and nonconducting regions of the fingerprint sensor, solves the moiré visibility problem at a fundamental level, as there is no interference with the pixel pattern of the display. Therefore, the key to the present study is to minimize the difference between the refractive index of the insulating and conducting regions of the sensor. In general, this insulating region is filled with materials having a refractive index similar to that of SiO2. Thus, if a conducting region with a similar refractive index can be provided, no visible pattern will be generated by the sensor, as light transmittance and reflectance in the two regions (insulating and conducting) are similar. To verify this idea, we simulated the optical properties of conventional ITO electrodes and OMO electrode thin-films using GSolver (Grating Solver Development Co.), a commercial software package. With ITO electrodes, the spectra of the light transmitted through the insulating regions is different from that of the light transmitted through the electrode area, as can be seen in Fig. 3(a). When the period of the fingerprint sensor’s electrode pattern is 70 µm, the transmission and reflection spectra differ significantly from the spectra obtained without an electrode pattern. In addition, when the size of the pattern is decreased, resonance characteristics can be observed at specific wavelengths. Since there is a difference between the refractive indexes of the insulation and electrode regions, the ITO electrode is optically recognized as a pattern, leading to the occurrence of the moiré problem when they are used as fingerprint sensors. In contrast, with the multilayer OMO electrode, there is not much difference between the spectrum of light transmitted through the insulating and conducting regions, as can be seen in Fig. 3(b). In addition, there is no significant difference between the transmission and reflection spectra, regardless of the size of the pattern period, meaning that it does not generate an interference pattern. Hence, the use of an OMO electrode, which does not produce an optical interference pattern, as a fingerprint sensor can be expected to solve the problem of moiré visibility.
To explain the moiré suppression phenomenon observed with the multilayer OMO electrodes, its structure should be considered. The electro-optical performance of these electrodes is described by an inverse relationship between electrical conductivity and optical transmission. As the thickness of the metal layer increases, the electrical conductivity of the electrode increases, while its optical transmittance decreases. In addition to increasing the optical absorption of the electrode, having too thick a metal layer results in strong visible light reflection, which cannot be effectively suppressed by the dielectric layer. Thus, the optimum thickness of the metal layer and the top oxide layer should be determined to ensure good electrical conduction. In addition to this, these thicknesses should be designed, together with the bottom oxide layer, to generate an antireflection effect, permitting high optical transmittance in the visible spectrum . Hence, the OMO electrodes used in this study were designed to consist of a 9-nm-thick Ag film, placed between 40-nm-thick Zn-doped In2O3 (IZO) films.
The prediction methods introduced earlier in this section can also be used to determine the moiré effect obtained when an OMO electrode with very high transparency is used. As seen in Fig. 4, the white dots present in the visibility circle when ITO is used as the electrode for the fingerprint sensor, disappear, or appear dim, when OMO electrodes with 93% transparency are employed. Hence, we predict that the moiré patterns are suppressed when OMO electrodes with transmittances greater than 93% are used.
3. Fabrication of fingerprint electrodes
ITO electrodes for fingerprint sensors can be manufactured using conventional photolithography and wet-etching processes. To increase the sheet resistance of the electrode, following ITO deposition, an annealing process is carried out at 250 °C for 1 h, in a furnace, with a working pressure of 10 mTorr, Ar flow rate of 1000 sccm, and oxygen concentration of 500 ppm. The annealing process reduces the sheet resistance of a 200-nm-thick ITO layer from 26.9 Ω sq−1 to 11.5 Ω sq−1.
Figure 5 illustrates a schematic of the process for fabricating OMO electrodes on a glass substrate. As mentioned above, this electrode consists of an IZO/Ag/IZO multilayer, each layer of which is sequentially deposited on a glass substrate. Inserting a nanosized Ag layer between the optimized IZO layers enables the control of the sheet resistance and optical transmittance of the multilayer. IZO (10 wt.% ZnO–90 wt.% In2O3) and Ag targets (99.999%) were used for deposition of the amorphous IZO layers and the metallic Ag layer, respectively. The 40-nm-thick IZO layers (top and bottom) were deposited on an 8-inch glass substrate at room temperature using the Centura 5200 tool (Applied Materials, Inc.) at a constant DC power of 1480 W, with an Ar flow rate of 200 sccm, O2 flow rate of 2 sccm, and working pressure of 8 mTorr. The Ag film was deposited using the SPS-TG RF sputtering tool (Ultech Co., Ltd.) at a power of 200 W, Ar flow rate of 60 sccm, and working pressure of 10 mTorr. The deposition rates were 1.56 and 0.156 nm s−1, for IZO and Ag, respectively. Next, we defined electrode patterns on the surface of the IZO/Ag/IZO multilayer, by exposing a photoresist, coated on the top of this film, to a 60 mJ cm−2 dose of radiation through a mask, and developing this exposed resist for 75 s. Unlike the ITO electrode, no separate annealing process is required to reduce the sheet resistance of the OMO multilayer, which is the biggest advantage of this particular fabrication process. The samples were then soaked in MA-PSW01 (Dongwoo fine-chem Co., Ltd.)—a H3PO4-based solution—for 1 min, to etch IZO/Ag/IZO film not coated with the developed photoresist. Following this, the samples were rinsed with deionized water, to remove the residual etching solution, and soaked in acetone for 10 min, and isopropyl alcohol for 5 min, to remove the photoresist. Figure 5(a) shows the result of etching the first IZO/Ag/IZO layer, revealing the pattern of the transmitting (Tx) electrodes of the fingerprint sensor. An organic insulating layer of DNI-LT09 (Dongwoo fine-chem Co., Ltd.)—an acryl polymer with an epoxy unit—was subsequently spin-coated onto this pattern, for 30 s at 500 rpm, and annealed at 80 °C for 85 s, as shown in Fig. 5(b). To create contact holes for electrical connections through the organic layer, the sample was subsequently exposed to a 90 mJ cm−2 dose of radiation, and developed for 80 s. Finally, the receiving (Rx) electrodes were manufactured on the organic insulating layer in the same manner as the Tx electrodes, i.e., the OMO deposition and etching process was repeated, followed by the inclusion of an organic passivation layer.
4. Results and discussion
As mentioned above, the thickness of a transparent electrode should be carefully selected, as both its sheet resistance and transmittance are independently controlled by this parameter. In particular, while increasing the thickness of the intermediate Ag layer decreases the sheet resistance of the OMO electrode, it is necessary to have as thin a layer as possible, while ensuring that a continuous Ag film is deposited, in order to improve its transmittance below the desired sheet resistance. In this study, we aimed for the sheet resistance of the OMO electrode to be similar to the sheet resistance of a 200-nm-thick ITO layer. The minimum Ag layer thickness corresponding to the deposition of a continuous film by our equipment was confirmed to be approximately 9 nm. Hence, we performed simulations to investigate the effect of varying the thicknesses of the upper and lower IZO layers on light transmittance and sheet resistance, under these conditions, as illustrated in Fig. 6. First, as shown in Fig. 6(a), optical simulation was carried out by fixing the thickness of the lower IZO and the thickness of Ag to 10 nm and 50 nm, respectively, and varying the thickness of the upper IZO. In Fig. 6 (b), the sum of the thicknesses of the upper and lower IZO layers was fixed to 100 nm, and the same optical simulation was performed. While the sheet resistance of the OMO electrode could be reduced to less than 8.5 Ω sq−1 with a 10-nm-thick Ag layer, satisfactory light transmittance characteristics could not be obtained, irrespective of changes in the thickness of the IZO layer. We noted the lowest sheet resistance, with excellent reflectance characteristics overall, when the upper and lower IZO layers were 50-nm thick. However, at wavelengths below 500 nm, the transmittance decreased sharply, and the reflectance increased as shown in Figs. 6(a) and 6(b). Although this phenomenon could be compensated by reducing the thickness of the Ag layer to 9 nm as in Fig. 6(c), doing so increased the sheet resistance of the entire OMO electrode to ~9 Ω sq−1. From the above results, it can be confirmed that the thickness of Ag greatly affects the optical characteristics and sheet resistance of the OMO electrode.
Figure 7(a) depicts the optical characteristics of ITO and OMO electrodes. From this image, we note a decrease in transmittance in the green wavelength band (480–550 nm) with the fabricated ITO thin film, which also has a large reflectance. This reduction in transmittance is reflected in the appearance of the fabricated electrode. It has been shown that the color of ITO thin films changes according to doping and oxygen concentration ; good ITO films are completely clear, while too much Sn turns the film tan and dark, and increasing the concentration of oxygen vacancies turns the films pale green and yellow, due to increased scattering. Our ITO film is pale green following electrode fabrication, suggesting increased scattering from oxygen vacancies. Conversely, in the green wavelength band, the OMO electrode has a transmittance of at least 90%, and a reflectance of less than 0.5%. Although this reflectance is similar to the result obtained from simulation, this is not the case with the transmittance. This discrepancy can be explained by the fact that in our simulation, absorption and scattering in the short wavelength region due to the surface roughness of the IZO thin films were assumed to be negligible. This assumption is only valid if the OMO layers are completely smooth, which is not the case with our electrode. Figures 7(b) and 7(c) illustrate a cross section of an OMO electrode fabricated to the optimal dimensions determined from simulation. These images illustrate that the Ag film in the first OMO multilayer is uneven, while the Ag layer in the second OMO multilayer is relatively uniform. The most probable explanation for the unevenness of the first Ag layer is Ag agglomeration, which is accelerated by a mismatch between the crystallographic structure of the substrate and the Ag thin film . This hypothesis is supported by Figs. 7(b) and 7(c). As the supporting IZO layer of the first OMO multilayer was deposited on a glass substrate, its surface roughness was slightly increased, increasing the irregularities in the thickness of the Ag film in this OMO structure. Conversely, due to the planarization effect caused by deposition on an organic insulating layer, the surface roughness of the supporting IZO layer of the second OMO multilayer is greatly reduced in comparison to the previous structure, improving the smoothness of the Ag layer.
To verify the moiré reduction effect of the OMO structure, we compared the interference generated with different electrode patterns varying in pitch and shape (rectangular, diamond, and fractal patterns were fabricated, as illustrated in the images below Fig. 8(c)), with the interference generated with ITO electrodes having the same structure. Figure 8(a) shows an array of two-layer ITO electrodes fabricated on a display panel. This array was configured such that the gaps in the different patterns varied in the x direction, and the periodicity of the different patterns varied in the y direction. The moiré phenomenon is typically suppressed by matching the periods of the overlapping pixel and electrode patterns. In this study, we adopted a PenTile structure for the display, with a pixel width of 44.5 µm. In Fig. 8(a), the electrode patterns in the red box have a period of 44.4 µm, similar to the display pixel period. While no specific moiré pattern is observable, we note the appearance of structural coloration. Thus, a variety of moiré phenomena can be seen even with a difference in period of 0.1 µm, the orientation of which appears even more clearly when the angle between the overlapping patterns is only 0.5°. We also observed changes in color depending on viewing angle and viewing distance. Hence, the pitch and angle tolerance are so small that moiré reduction through period matching is difficult. By using this result, we selected the pattern period and angle that generated the least prevalent moiré phenomenon, for further investigation, noting that modifying the shape of the pattern did not affect this phenomenon. Figures 8(b) and 8(c) depict a one-layer ITO electrode and a one-layer OMO electrode, respectively, both fabricated using the selected pattern configuration. In the case of the ITO electrode, moiré patterns are still observed. However, moiré patterns are almost completely suppressed with the OMO electrode. In particular, the most clearly observable moiré pattern generated by the ITO electrode is not created with an OMO electrode in the same configuration, as highlighted by the red boxes in Figs. 8(b) and 8(c), respectively. The moiré patterns highlighted by the blue boxes in Figs. 8(b) and 8(c) (labelled 1 and 2 for the ITO and OMO electrodes, respectively) also illustrate further differences between the optical behaviors of the different electrodes. With the ITO electrode, no moiré pattern is visible at reduced magnification. However, enlarging the image reveals an underlying periodicity. This effect is not observed in the case of the OMO electrodes.
Following this investigation, we fabricated two-layer ITO and OMO electrodes, and observed the prevalence of the moiré phenomenon as the orientation between the display and the electrodes was varied, as shown in Fig. 9. By comparing the array of two-layer ITO electrodes depicted in Fig. 9(a) with the results in Fig. 8(b), an overall improvement in the appearance of the moiré phenomenon can be observed. In particular, the moiré patterngenerated by the electrode configuration in the red box, which was previously the most prevalent pattern, has been improved. This suppression is a result of the two-layer fabrication process, as the area of the insulating layer with a refractive index differing from that of the ITO electrode region is reduced, as shown in Fig. 5(d). However, it should be noted that invisible moiré patterns reappear when the angle between the electrode and the display is modified; moiré patterns that were not observed at a 10° rotational angle were observed at 16°. It clearly shows that the moiré phenomenon is observed with all electrode configurations when this orientation effect is considered. In contrast, two-layer OMO electrodes do not exhibit a visible moiré phenomenon even when the orientation is rotated arbitrarily. However, we note that even with these electrodes, the moiré phenomenon is not completely removed at all angles. This deviation from the predicted behavior was anticipated from Fig. 7(a), where the measured transmission characteristics differ from the result of simulation.
As human eyes have a noted sensitivity to the color green, the optical behaviors of the different electrodes at related wavelengths can be used as a predictor of the efficacy of the suppression technique for actual displays. At 540 nm (corresponding to the color green), the difference in transmittance between the ITO electrode and the surrounding insulating layer is ~16%. This difference is ~6% with the OMO electrode, explaining the considerable reduction in the appearance of the moiré phenomenon. Hence, although the OMO electrode is separated into non-conducting and conducting regions, because the difference in the refractive indexes of the two regions is small, the appearance of the moiré phenomenon is suppressed, as predicted by the analysis conducted in Section 2.
Finally, we fabricated a fingerprint sensor with OMO electrodes. Figures 10(a)-10(c) show the Tx and Rx electrode regions, electrode contact region, and electrode pad region of the fingerprint sensor, respectively. The pad and the fingerprint sensor electrodes were connected through molybdenum electrodes. Figure 10(d) shows the contact resistance measurements for the electrical test element group (TEG) patterns, configured to determine whether the fingerprint sensor is operating. The inset to this image illustrates the electrode. The results depicted in this image show that the resistance of the OMO electrode is approximately half (~7 Ω sq−1) of that of the ITO electrode, which has already been deployed in fingerprint sensors, confirming that in this respect, OMO electrodes are suitable for fingerprint sensors.
In this study, we have developed an on-screen fingerprint sensor based on multilayer OMO electrodes, with which the appearance of moiré patterns is suppressed. By analyzing the moiré phenomenon in the frequency domain, we proved multilayer OMO electrodes to be theoretically capable of minimizing the moiré phenomenon, as the small difference in transmittance and reflectance between this structure and the surrounding insulating layer meant that it did not generate an optical interference pattern. To create this geometry in practice, we designed an IZO/Ag/IZO multilayer where the transmittance was maximized and the reflectance was minimized in the visible light region, and the sheet resistance of the electrode was optimized. Hence, we obtained a moiré-free fingerprint sensor with a transmittance of at least 90% at 540 nm, and a low resistance of ~7 Ω sq−1. Our experiments verified that, unlike conventional ITO electrodes, the proposed two-layer OMO electrode greatly reduces the undesirable moiré phenomenon, regardless of pattern shape and period. More importantly, when the electrode was rotated on the display panel, the moiré phenomenon was remarkably reduced, compared with the patterns generated by the ITO electrodes. This approach for creating moiré-free transparent conductive electrodes can thus provide a viable alternative to existing ITO electrodes, and facilitate the development of new optoelectronic devices.
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