IR sensitivity enhancement of CMOS Image Sensor with diffractive light trapping pixels.

Area of Science:

• Optoelectronics
• Materials Science
• Nanotechnology

Background:

• Back-illuminated CMOS Image Sensors (BI-CIS) are crucial for advanced imaging.
• Enhancing near-infrared (NIR) sensitivity in BI-CIS remains a key challenge.
• Current BI-CIS designs often have limitations in capturing longer wavelengths.

Purpose of the Study:

• To investigate the IR sensitivity enhancement of BI-CIS using a 2D diffractive inverted pyramid array (IPA) structure.
• To quantify the improvement in light absorption and sensor sensitivity at specific IR wavelengths.
• To evaluate the practical application of this technology in real-world imaging scenarios.

Main Methods:

• Finite-difference time-domain (FDTD) simulations were performed on crystalline silicon (c-Si) with 2D IPAs.
• Fabrication of a prototype BI-CIS with a 1.2 μm pixel size and 400 nm pitch IPAs.
• Experimental testing of the prototype BI-CIS using a demo camera and C-mount lens.

Main Results:

• Simulations predicted over 30% light absorption improvement at 850 nm with IPA pitches over 400 nm, peaking at 43% with a 540 nm pitch.
• The prototype BI-CIS demonstrated an 80% sensitivity enhancement at 850 nm compared to a flat surface reference.
• The demo camera achieved a 75% sensitivity enhancement in the 700-1200 nm range with negligible spatial resolution degradation.

Conclusions:

• The 2D diffractive IPA structure effectively enhances IR sensitivity in BI-CIS through diffraction and total internal reflection.
• This light-trapping CIS pixel technology offers a significant improvement in NIR sensitivity.
• The technology is applicable to diverse imaging applications, including security cameras, authentication systems, and Time-of-Flight cameras.