Te blue curve shows the horizontal angular dependence of the relative quantum efficiency of the interline transfer CCD image sensor. From an incident angle of 5° and higher, the relative quantum efficiency drops significantly, while the vertical dependence is less pronounced, which can be explained by the fact that in vertical direction the light sensitive area nearly covers the whole pixel while horizontally half or more of the area is used by the shielded register.

FROM FRONTSIDE TO BACKSIDE ILLUMINATION However, the micro-lenses cannot collect and focus all ray angles of incoming light. Further, the semiconductor manufacturing process contributes additional layers above the photodiodes (figure 3, depicting the wiring, the transistors and the capacitors). Te electronics in these layers can cause light to be scattered and absorbed, resulting in a loss of light to charge conversion. Te loss of light due to physical blocking and scattering by electronics is more pronounced in CMOS image sensors with small pixel pitches and higher pixel counts (>4 MP) than many CCD sensors. Due to massive adoption of CMOS sensors (e.g. smartphone cameras) semiconductor manufacturers have developed methods to process the wafer with the image sensors effectively reversed, and a large part of the substrates physically and chemically etched away. Tis process results in image sensors that are effectively illuminated from the back, and the light reaches the photodiodes more directly (Fig. 3b).

Fig. 3. Schematic cross section through a frontside illuminated CMOS image sensor with micro lenses (a) and a back- side illuminated CMOS image sensor with micro lenses (b) to illustrate the effect of frontside versus backside illumination

Backside illumination of current CMOS image sensors has seen quantum efficiencies better than 90%. By the introduction of an additional surface (the surface of the backside), there are also additional dark current and noise sources added, the caveat being that many backside illuminated image sensors have higher dark current compared to the frontside illuminated counterparts.

BACKSIDE ILLUMINATED IMAGE SENSORS WITH MICROLENSES Te advantage of having fewer layers above the photodiodes (higher sensitivity) also presents a disadvantage in decreased sharpness – technically described as modulation transfer function (MTF). Due to the remaining substrate above the photodiodes, backside illuminated image sensors generally show a decreased MTF,

and if light arrives at particular angles, can be scattered or incorrectly guided to the next pixel. Luckily, the same microlens method, initially developed to increase the fill factor, now improves the MTF. Fig. 4 illustrates the light rays hitting a backside illuminated image sensor under an angle (Fig. 4a), and showing that the microlenses (Fig. 4b) help collect the light at the photodiodes belonging to the pixel, where the light was impinging. Conversely, as mentioned above in

Fig. 2, the introduction of microlenses again has an impact on the angular dependence of the quantum efficiency, which means that the back illuminated image sensors without microlenses have a larger independence of the incident angle, even better than the red curve in Fig. 2.

THE ANSWER Returning to the answer of our initial question, backside illuminated image sensors have fewer obstacles in the pathway of the incoming light as it reaches the volume of the pixel, where the conversion to charge carriers takes place. Terefore, backside illuminated CMOS image sensors are able to convert more of the light into charge carriers, resulting in larger signals and better images.

Fig. 4. Schematic cross section through a backside illuminated CMOS image sensor without (a) and with microlenses (b) to illustrate the microlens effect of improving the MTF

Dr Gerhard Holst is Head of Science & Research Business Development at PCO. xx 69

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