Studies on the properties of integrated CMOS image sensor arrays on single crystalline thinned and flexible silicon chips

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Silicon based CCD and CMOS image sensors are ubiquitous nowadays and are employed in different areas like cameras and cell phones. These sensors differ in size and resolution depending on their application but because of production reasons all are planar and rigid because of their thickness of 500-800 µm. However, only a layer of some micrometers on the top of the chip is necessary for the sensory function so that by removing of material of the backside to a remaining thickness of below 30 µm they become mechanically flexible. Thereby it is possible to apply them to a cylindrical surface by what a 360° angle view sensor can be implemented. This would be of great interest for applications in endoscopy.In the course of the first project phase extensive experience in circuit design, chip thinning, electrically and mechanically contacting of thinned chips and in measurement techniques has been gained. The influence of mechanical stress caused by bending of thinned sensors on several devices - as photo diodes, transistors and capacitors - under different substrate orientation was systematically examined, based thereon circuit concepts for the minimization of the bending dependence were developed and their suitability was verified using a pixel read-out circuit employing these concepts. In the course of the research new questions have arisen and a clarification will allow for an extensive comprehension of the behavior and of the optimal design of CMOS integrated image sensors on flexible substrates subjected to bending. Firstly, the influence of mechanical stress on the noise characteristics of photo diodes and transistors shall be studied as noise is limiting image sensor performance especially under low light conditions. Furthermore it shall be examined if a bandgap reference voltage source that is optimized regarding its voltage stability under mechanical stress can be designed by employing the design rules developed in the first project phase because the supply of a stable reference voltage is necessary for the functioning of many analog circuits. Furthermore, it shall be examined if the stress dependence of transistors can be reduced by modifying the gate and channel geometry and what influence the chip thickness has on the photodiode dark current. The suitability of thinned and flexible image sensors for new camera concepts shall be confirmed on an endoscope like demonstrator, where the sensor is mounted on the outside of a cylindrical carrier.