Fingerprint Sensor Side-Lit Light Guide Total Internal Reflection
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Solution Overview
Problem
Existing fingerprint recognition sensors face challenges with low light efficiency, increased thickness and volume, reduced transparency, and complex manufacturing processes, which hinder their integration with other devices and reliability in media other than air.
Innovation Solution
A fingerprint recognition sensor design featuring a backlight unit with a base layer and a light source positioned on one side, a cover layer, and sensor pixels with photo sensors that utilize light with incident angles greater than the critical angle for enhanced light transmission and reflection, eliminating the need for structures like prisms or reflection plates, thereby reducing thickness, volume, and manufacturing complexity.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Illumination intensity
If a backlight unit is positioned on the bottom of the substrate, then light can be irradiated to sense the fingerprint, but transparency of the sensor is reduced and coupling with other devices becomes difficult
Solution Approach 1:
The patent inverts the conventional backlight positioning from the bottom of the substrate to the side of the substrate. This inversion allows the light source to irradiate light through the side surface, eliminating the need for a bottom-positioned backlight unit that blocks transparency. The light guide layer then directs this light through the fingerprint contact surface, achieving both effective illumination and maintained transparency.
2Ease of manufacture
If a black matrix pattern is used to restrict light angle, then light can be directed for sensing, but the amount of light incident on the fingerprint surface is small and light use efficiency is low
Solution Approach 1:
The patent replaces the black matrix mechanical structure with an optical approach using a light guide layer. Instead of using opaque black matrix patterns to restrict light angles, the light guide layer utilizes optical total internal reflection to direct light at precise angles. This substitution eliminates light absorption by the black matrix and significantly improves light use efficiency while maintaining effective light direction control.
Solution Approach 2:
The patent changes the optical parameters by utilizing total internal reflection at specific incident angles greater than the critical angle. By controlling the light incident angle parameter and the refractive index parameters of the light guide layer, the system achieves efficient light direction without the need for black matrix absorption, thereby improving light use efficiency.
3Illumination intensity
If prism structures are introduced on the bottom of the substrate, then light can be refracted toward the cover, but the thickness and volume of the sensor increase and transparency decreases
Solution Approach 1:
The patent extracts and eliminates the prism structure from the sensor design. Instead of incorporating bulky prism structures on the bottom of the substrate to achieve light refraction, the invention uses a side-positioned light source combined with a light guide layer that utilizes total internal reflection. This extraction of the prism structure significantly reduces the sensor's thickness and volume while maintaining effective light direction.
4Ease of manufacture
If reflection plates or prism structures are positioned in the optical path, then light can be directed, but the thickness and volume of the sensor increase
Solution Approach 1:
The patent merges the light direction function into the light guide layer itself, eliminating the need for separate reflection plates or prism structures. The light guide layer is designed with specific refractive index characteristics that enable total internal reflection, combining the functions of light guidance and light direction control within a single thin layer, thereby minimizing the sensor's overall volume.
Applied Scientific Principles
This section explains which scientific principles are used to turn an abstract innovation direction into a practical engineering solution.
Function Achieved in This Case
This design enhances light efficiency, improves fingerprint recognition reliability in various media, simplifies manufacturing, and facilitates integration with other devices by reducing thickness, volume, and manufacturing costs while maintaining transparency.
Implementation Method 1
light of which an incident angle on a surface facing the substrate is larger than a critical angle in the light irradiated from the light source is transmitted toward the substrate
Data Source
AI summary
Disclosed is a fingerprint recognition sensor including: a backlight unit including a base layer positioned on one surface of a substrate and including a transparent material and a light source irradiating light to the inside of the base layer on one side of the base layer, in which light in which an incident angle on a surface facing the substrate is larger than a critical angle is transmitted toward the substrate; a cover layer spaced apart from the substrate and facing the other surface of the substrate; multiple sensor pixels defined by multiple scan lines and multiple data readout lines on the other surface of the substrate and positioned between the substrate and the cover layer; and multiple photo sensors positioned in the respective sensor pixels and sensing light transmitted on the base layer toward the substrate and total-reflected on a surface contacting a fingerprint of a user on the cover layer.