Crosstalk and microlens study in a color CMOS image sensor

The paper describes results of crosstalk investigations and microlens (/spl mu/-lens) scan experiments in a color CMOS image sensor with active pixel structure . The investigation of optical and electrical crosstalk was made on 7.8- and 5.6-/spl mu/m pixels by using samples with continuous shift of color filter (CF ) and /spl mu/-lens across the array. As a result of this investigation, the distribution of sensitivity inside a pixel has been determined. By using minimum crosstalk criteria, the optimum parameters of the /spl mu/-lens manufacturing process and optimum position of the /spl mu/-lens was determined. The paper presents color maps of pixel sensitivity and crosstalk criteria as well as snapshots illustrating sensitivity distribution and collection area. The paper presents spectral characteristics measured at different relative apertures (f-number) as well. The quantitative analysis of spectral responses allowed us to determine the contribution of each component to the overall crosstalk.

I. Introduction

Crosstalk in an image sensor array degrades the spatial resolution, reduces overall sensitivity, makes poor color separation, and leads to additional noise in the image after color correction procedure. We will consider crosstalk in CMOS image sensors as consisting of three main components. •

Spectral crosstalk: this component is due to imperfect color filters passing through some amount of unwanted light of other colors.

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Optical spatial crosstalk: the main reason for this component of crosstalk is that color filters are located at some distance from the pixel surface due to metal and insulation layers. The light coming at angles other than orthogonal passes through a filter and can partially be absorbed by the adjacent pixel rather than one below. Depending on the -number of the lens, this portion of the light absorbed by neighboring pixel can vary significantly and can be big enough for low -numbers. -lenses located on the top of color filters reduce this component of crosstalk significantly when appropriate form of -lenses and optimum position of them are chosen. Fig. 1. Image under green light illumination. “Color separated” view. Green 1 and Green 2 pixels—first and third quarters; red and blue pixels—second and fourth quarters.

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Electrical crosstalk: this component of crosstalk results from photo-generated carriers having the possibility to move to neighboring charge accumulation sites. In comparison to the previous two components, this crosstalk occurs in both monochrome and color image sensors. The quantity of the carriers that can be accumulated by the neighboring pixel and the corresponding crosstalk strongly depends on the pixel structure, collection area, and distribution of sensitivity inside a pixel.