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MICROSCOPY & IMAGING


ILLUMINATING ANSWERS G


ermany-based camera manufacturer PCO develops scientifi c CMOS (sCMOS) cameras with both, back and front illuminated sensor technology. When comparing back with front illuminated sensors, one question is often raised: why is a backside-illuminated sensor more sensitive than a front side illuminated? All image sensors have light-sensitive


pixels, but what does that mean? T e pixels allow a spatial localisation of incoming light and consist of various electronics interconnected with metal wires to enable digitisation. T e basic light to charge carrier conversion elements are the photodiodes. Both the lateral area and volume of a pixel are shared by photodiodes, metal wiring, transistors and capacitors. T us, the sensitivity of an image sensor strongly depends on how much of the total pixel area is used for light to charge carrier conversion, or in other words, is light sensitive.


FILL FACTOR T e fi ll factor is a technical term for an image sensor, which describes the ratio of light sensitive area to total area of a pixel:


fi ll factor = (light sensitive area of pixel) (total area of pixel)


For example, in interline transfer CCD image sensors where the pixel area was shared by the photodiode and the shielded register, the fi ll factor was in the range of 30%. T is means, a minimum of 70% of the incoming light would have been lost. T e same principle holds true for CMOS image sensors, where all the transistors, capacitors and wires occupy valuable light converting space. During CCD sensor development, measures were developed to compensate the fi ll factor


68 www.scientistlive.com


loss. T e most eff ective measure was done simply by adding micro lenses on top of the image sensor. Fig. 1 illustrates the diff erences in light collection shown for perpendicular impinging light. While some of the light is scattered, refl ected or absorbed in spaces of the image sensors, the microlenses focus the light to the charge conversion photodiodes much more effi ciently than without (Fig. 1a and 1b). By this measure, the CMOS image


sensor, shown in Fig. 1, has a total quantum effi ciency of about 50% – quite good considering there are additional loss mechanisms in image sensors. T e best quantum effi ciencies achieved in interline transfer CCDs (with 30% fi ll factor) have been around 50 to 70%. In more recent sCMOS image sensors with similar fi ll factors, quantum effi ciencies of above 80% have been achieved by optimisation of the microlenses and the manufacturing process. But the microlenses are, in most cases, made of mouldable plastic material, which attenuates signifi cantly any UV


Dr Gerhard Holst tackles a common question in the discussion about back- versus front-illuminated sensors


Fig.1. Schematic cross section through a frontside illuminated CMOS image sensor without (a) and with microlenses (b) to illustrate the microlens eff ect of improving the light collection


light transmission. Furthermore, there is a new infl uence


introduced by microlenses, since the performance of these optical devices is dependent on the angle of incidence. T is means that the microlenses add a more pronounced angular dependency to the quantum effi ciency, as can be seen in Fig. 2.


Fig. 2. Measured data of the vertical and horizontal angular dependence of the relative quantum effi ciency of a interline transfer CCD (KAI-2020) with micro lenses


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