Image sensors are devices found in everyday life converting light or radiation to image or digital data. The most common application is found from digital mobile phone cameras, but image sensors are also having important use in medical, military, surveillance and industrial applications.
ALD is used in image sensors mainly as a passivation layer. Wide selection of materials and the possibility to tune the process enable creation of layers that have the desired electrical and optical properties. For example, surface charges critical for the device performance, can be tailored by mixing two or more ALD materials or by tuning the used chemistry or deposition conditions.
ALD layers can also be used as anti-reflection layers to improve the amount of radiation collected by the sensor as films with different refractive index can be stacked in a single process run.
As in all semiconductor industries, also in image sensors there is an ongoing race to reduce the device/pixel size and to improve the general device performance. One main step in this process is the introduction of deep trench isolation. In back deep trench isolation technology, the trench is coated with high-k material made by ALD. The selected high-k material has negative charge, which increases hole accumulation at the interface of the layer and the substrate. The depletion region suppresses dark-current, which is the current flowing in the sensor even without the presence of light. ALD processes superior conformality and thickness control. Together with dielectric materials well adopted by the semiconductor industry, they have made image sensor manufacturing transition from using CVD to ALD.