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

硅基CCD和CMOS图像传感器如今无处不在，并被应用于相机和手机等不同领域。这些传感器的尺寸和分辨率因其应用而异，但由于生产原因，它们都是平面和刚性的，因为它们的厚度为500-800 μ m。然而，在芯片的顶部只需要几微米的层就可以实现感应功能，因此通过去除背面的材料，使其剩余厚度低于30微米，它们就变得具有机械柔性。因此，有可能将它们应用于圆柱形表面，通过360°角度视图传感器可以实现。这对内窥镜检查的应用具有重要意义。在第一个项目阶段的过程中，在电路设计、芯片减薄、减薄芯片的电气和机械接触以及测量技术方面获得了广泛的经验。系统地研究了在不同衬底取向下，薄传感器对光电二极管、晶体管和电容器等器件弯曲引起的机械应力的影响，在此基础上提出了最小化弯曲依赖的电路概念，并使用采用这些概念的像素读出电路验证了它们的适用性。在研究过程中出现了新的问题，澄清将允许对柔性基板上CMOS集成图像传感器的行为和优化设计有广泛的理解。首先，需要研究机械应力对光电二极管和晶体管噪声特性的影响，因为噪声限制了图像传感器的性能，特别是在弱光条件下。此外，还应检查在机械应力下电压稳定性方面进行优化的带隙参考电压源是否可以通过采用第一个项目阶段开发的设计规则来设计，因为稳定的参考电压的供应对于许多模拟电路的功能是必要的。此外，还应检查是否可以通过修改栅极和沟道几何形状来降低晶体管的应力依赖性，以及芯片厚度对光电二极管暗电流的影响。薄型和柔性图像传感器对新相机概念的适用性应在类似内窥镜的演示器上得到确认，其中传感器安装在圆柱形载体的外部。

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.