Dramatic Reduction of Optical Crosstalk in Deep-Submicrometer CMOS Imager With Air Gap Guard Ring

Abstract

A novel dielectric structure, air gap guard ring, has been successfully developed to reduce optical crosstalk thus improving pixel sensitivity of CMOS image sensor with 0.18-m technology. Based on refraction index (RI) differences between dielectric films (RI = 1 4 1 6) and air gap (RI = 1), total internal reflection occurred at dielectric-film/ air-gap interface, thus the incident light is concentrated in selected pixel. Excellent optical performances have been demonstrated in 3.0 3.0 m pixel. Optical spatial crosstalk achieves 80% reduction at 20 incidence angle and significantly alleviates the pixel sensitivity degradation with larger angle of incident light.

A novel method to test and optimize the periphery crosstalk in CMOS image sensor

“…In the pixel structure, including shifted microlens [8] and zero gap microlens [9], plasmonic color filter [10,11,12,13] and lensed color filter [14], optimized light guidance structure [15,16]. As for the process development, deep p-well around photodiode [17], deep trench isolation (DTI) [18] including frontside-illumination DTI [19], backside-illumination [20] and capacitor DTI [21], air-gap guard ring [22,23,24,25], anti-reflection film coating [26] and buried shield metal [27] are all applied to optimize the crosstalk behavior respectively. PMOS structure [28] also make sense due to its limit in diffusion.…”

Optical Performances of Lensless Sub-2micron Pixel for Application in Image Sensors

“…This is mainly related to the fact that the technological processes employed in the large scale device production do not allow, at present, a further reduction of both the thickness of the optical stack hosting the metal lines used for the electrical connections and the distance between adjacent pixels. On the other hand, from the point of view of the physical mechanisms, it has been recently demonstrated that for pixel size smaller than 1.4 µm, diffraction effects become so significant to limit the effectiveness of the microlens focusing properties [1] and this, in turn, produces large optical crosstalk effects between adjacent pixels that are significant also for pixel size close to 3.9 µm [2]. Advances in material research make now possible to exploit the possibility to replace front-side illuminated image sensors with the back-side illuminated counterparts in order to reduce the pixel thickness at least for what concerns the optical path and increase the optical efficiency and the device fill factor [3,4].…”