In addition, as these sensors can be easily fabricated on a polymer film substrate, they are better suited for foldable smartphones compared to other types of sensors. In addition, these sensors can work on both OLED (organic light-emitting diodes) and LCD (liquid crystal display) screens, while optical and ultrasonic on-screen sensors can work with OLED displays only. Mutual-capacitive-type on-screen fingerprint sensors can be integrated with touch sensors in the same substrate layer, and fabricated on a large area (>200 mm 2) without considerable additional cost. The mutual capacitive-type on-screen fingerprint sensor is another promising candidate because its not-on-screen version has been employed in a large number of commercial smartphones, and its performance has been confirmed. Ultrasonic under-display fingerprint sensors also have room for improvement in terms of recognition time, dry finger recognition, and manufacturing yield 10. This issue arises because the sensor may identify a finger ridge area, which is not making good surface contact, as a valley area since part of the light will be reflected at the cover surface instead of propagating through the finger on the surface. The optical under-display fingerprint sensor, which recognizes a fingerprint using the difference in light reflected at the finger ridge and valley areas, has an issue in recognizing dry fingers, which cannot make uniform and consistent contact with the surface of the sensor cover layer 12, 13. However, in some cases, these sensors may not work as satisfactorily as conventional fingerprint sensors. Among them, optical and ultrasonic sensors are used in some smartphone models because they can be placed under the display and do not interfere with display imaging, even though they are relatively bulky and costly. Typical examples include capacitive, optical, and ultrasonic fingerprint sensors. Various types of sensors that can recognize fingerprints on display screens have been developed 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11. However, the most convenient and intuitive location of the fingerprint sensor is on the front face. For this type of smartphone, fingerprint sensors, formerly located under the home button, have to be positioned on the back or side of the smartphone. By applying the selected sensor pattern and circuit driving by block, fingerprint sensing on a display is demonstrated with a prototype built on a commercial smartphone.įull-screen smartphones with extremely thin bezels without physical buttons on the front have become the mainstream in the smartphone industry in recent years. As the selected sensor pitches are too small to detect capacitance variations, three unit patterns are electrically connected to obtain a unit block generating a larger signal. The range is narrowed for an experimental evaluation, which is used to finally determine the sensor design. To search for appropriate patterns, a numerical calculation is carried out over wide ranges of pitches and rotation angles. It is necessary to find an appropriate sensor pattern that minimizes the Moiré pattern, while maintaining the signal sensitivity. The interference between periodic display pixel arrays and sensor patterns can lead to the Moiré phenomenon. Even at this high transmittance, the electrodes can degrade the display quality when they are placed on the display. and a transmittance of ~94% (~86% with the substrate effect) in the visible wavelength range, and assembled onto a display panel. On-screen fingerprint sensors are fabricated using an indium tin oxide transparent conductor with a sheet resistance of ~10 Ω/sq. In this study, a mutual capacitive-type on-screen fingerprint sensor, which can recognize fingerprints on a display screen to provide smartphones with full-screen displays with a minimal bezel area, is fabricated.
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