# A High Full Quality CMOS Image Sensor for Space Applications
###### Abstract  
This paper presents a high full well capacity (FWC) CMOS image sensor (CIS) for space applications. The proposed pixel design effectively increases the FWC without inducing overflow of photo-generated charge in a limited pixel area. An MOS capacitor is integrated in a pixel and accumulated charges in a photodiode are transferred to the in-pixel capacitor multiple times depending on the maximum incident light intensity. In addition, the modulation transfer function (MTF) and radiation damage effect on the pixel, which are especially important for space applications, are studied and analyzed through fabrication of the CIS. The CIS was fabricated using a 0.11 \(\upmu\)m 1-poly 4-metal CIS process to demonstrate the proposed techniques and pixel design. A measured FWC of 103/448 electrons and MTF improvement of 300% are achieved with 6.5 \(\upmu\)m pixel pitch. CMOS image sensors; wide dynamic range; multiple charge transfer; space applications; radiation damage effects 1  
W.-T.K. The DR is a quantity that describes the range of light intensity in an image. A high-DR image sensor can produce images with low to high light levels in an exposure time. It is defined as the ratio between the maximum saturated pixel output level and noise level in the dark [4]. The DR can be calculated as  
\[\text{Dynamic range }(\text{DR})=20\log\biggl{(}\frac{\text{N}_{\text{sat}}}{ \text{N}_{\text{dark}}}\biggr{)}[\text{dB}], \tag{1}\]  
where N\({}_{\text{sat}}\) is the number of electrons collected by a pixel at saturation level, which is also referred to as full well capacity (FWC), and N\({}_{\text{dark}}\) is the number of electrons at noise level without illumination. The improvement of DR is an important issue not only for image quality but also for accuracy of space sensor applications [5; 6]. For example, star trackers use known star positions in an image taken with CISs on a spacecraft. An algorithm of the star tracker finds centroids of stars by using blurred spots. Because the wide dynamic range (WDR) of CIS makes the blurred spot more informative, the accuracy of the star tracker is increased with rapid determination of centroids [7]. Methods for achieving WDR can be categorized as a non-linear or linear. Previous reports have described DR extension with non-linear methods by several means. A well capacity adjusting method controlling an overflow gate caused low SNR because of the non-linear response of charge collection [8]. A logarithmic active pixel sensor using logarithmic response of MOS transistor in a weak inversion region caused high fixed pattern noise (FPN), which limits SNR because of large threshold voltage variation [9]. Linear-logarithmic pixel was also introduced to compensate the degradation of SNR, but the pixels were based on the three-transistor (3T) pixel structure, which has poor noise performance compared to a four-transistor (4T) pixel structure [10]. The linear method for the WDR is to increase FWC physically. In previous works [11; 12], the FWC is required to be larger than 75,000 e\({}^{-}\) for space applications. To satisfy this requirement, large pixel pitch was simply adopted. If high resolution is required, the chip size of CISs becomes large, so the manufacturing cost of the image sensor is increased and optical components also become bulky, which is not recommended for small satellite applications. Therefore, a demand for high FWC with small pixel pitch has been raised. Modulation transfer function (MTF) is another important specification for quantifying image quality. It defines the ability of an optical system to resolve a contrast at spatial frequency. Higher MTF makes small objects distinguishable on images. For example, earth observation satellites, which observe geographical features, weather, and surveillance, require high MTF to distinguish over several meters or centimeters. The major cause of MTF degradation is the crosstalk between neighboring pixels, especially for small-pitch pixels [13]. There can be both electrical and optical crosstalk depending on the cause.