The increasing popularity of full-screen smartphones necessitates the integration of fingerprint sensors directly onto the display. While optical and ultrasonic under-display sensors exist, they have limitations such as poor dry finger recognition (optical) and issues with recognition time and yield (ultrasonic). Mutual capacitive on-screen fingerprint sensors offer an attractive alternative due to their compatibility with both OLED and LCD screens, cost-effectiveness, scalability, and suitability for foldable phones. However, the placement of the sensor on the display raises concerns about image quality degradation, particularly the appearance of Moiré patterns due to the interaction between the sensor's electrode pattern and the display's pixel array. This paper addresses the challenge of designing a sensor pattern that minimizes Moiré patterns while maintaining adequate sensitivity.

The paper reviews existing fingerprint sensor technologies, highlighting the advantages and disadvantages of capacitive, optical, and ultrasonic sensors. It emphasizes the limitations of optical sensors in recognizing dry fingers and the shortcomings of ultrasonic sensors concerning recognition time, dry finger recognition, and manufacturing yield. The authors position the mutual capacitive on-screen fingerprint sensor as a superior alternative due to its performance, cost-effectiveness, and compatibility. The literature review also discusses previous work on Moiré pattern reduction in touch screens, noting that the smaller electrode pitches required for fingerprint sensors pose a more significant challenge than those encountered in touch screens.

The researchers used numerical calculations and experimental observations to determine the optimal sensor electrode pattern. They employed discrete Fourier transforms (DFT) on grayscale images of overlapping display and sensor patterns to analyze spatial frequencies and predict Moiré pattern characteristics. The amplitude of the DFT results, representing the perceptual contrast of the Moiré pattern, was combined with a human contrast sensitivity function (CSF) to quantify approximate Moiré visibility (MV). This MV value, while not perfectly accurate due to the limitations of simple summation of frequency components, served as a criterion for selecting candidate patterns. Subsequently, 55 different ITO electrode patterns were fabricated and evaluated on a glass wafer, and later on a smartphone display. High-resolution images of the Moiré patterns were obtained under controlled conditions for objective comparison. Finally, the best-performing pattern, selected based on MV calculations and visual assessment, was integrated into a prototype smartphone to demonstrate its functionality.

The DFT and MV calculations identified ranges of sensor pitches and rotation angles that minimized Moiré pattern visibility. The analysis revealed that patterns with smaller pitches, around 30 µm or smaller, produced less visible Moiré patterns when rotated at specific angles (7–12°, 15–20°, and 35–40°). Experimental evaluation confirmed these findings, with high-resolution images showing significantly reduced Moiré patterns for the selected patterns compared to a standard interlocking diamond pattern. Even though the individual sensor pitches are too small for sufficient signal generation, connecting three unit patterns electrically created larger unit blocks, which improved the fingerprint detection capability. The prototype smartphone successfully demonstrated fingerprint image acquisition using the optimized sensor pattern and block-wise circuit driving.

The successful demonstration of a functional on-screen fingerprint sensor with minimized Moiré patterns addresses a significant challenge in the field of smartphone technology. The methodology of using DFT, MV calculations, and high-resolution image analysis provides a robust framework for optimizing sensor patterns. The findings contribute to the development of more aesthetically pleasing and user-friendly full-screen smartphones. Future research could focus on further refinements of the sensor design to improve sensitivity, reduce power consumption, and explore alternative transparent electrode materials beyond ITO.

This study successfully designed and fabricated a mutual capacitive on-screen fingerprint sensor that minimizes Moiré pattern interference with the display. The use of DFT and MV calculations in conjunction with experimental validation led to the identification of an optimal sensor pattern. This work paves the way for the widespread adoption of fully integrated fingerprint sensors in high-resolution mobile displays. Future work might explore advanced materials and patterning techniques to further improve sensor performance and transparency.

The MV calculation, based on simple summation of spectral components, is an approximation and might not fully capture the subjective perception of Moiré patterns. The study focused on a specific smartphone display model; further investigation is needed to determine the generalizability of the optimized patterns to different displays. The study also considered primarily visual aspects of Moiré patterns; other aspects like their potential influence on display color accuracy might merit further exploration.