[Preparation and spectral characterization of Lumogen coatings for UV-responsive CCD image sensors]

Traditional charge-coupled devices (usually front-illuminated CCDs) and complementary metal oxide semiconductor (CMOS) have lower response in ultraviolet region particularly. The reason is that polysilicon gate material absorbs the ultraviolet radiation highly, which leads to a barricade of the radiation penetrating the gate to the channel of CCD. To enhance the detective responsibility of CCD in the ultraviolet region, a feasible method is to coat the surface of CCD polysilicon gate with a thin film. The thin film should have the ability of converting the ultraviolet to visible in order to enable the UV radiation to "penetrate" the polysilicon gate. An organic coating to convert the UV radiation to visible has been developed in the present paper. Lumogen thin films were deposited on fused silica substrates by vacuum evaporation of an organic dye called Lumogen Yellow S0790. Analysis of organic functional groups was used to study the luminescence mechanism of Lumogen. The optical constants of coatings were calculated by spectroscopic ellipsometry. The results indicate that Lumogen exhibits photoluminescence continuously owing to four kinds of double bonds in each Lumogen molecule. The refractive index of Lumogen film was approximately 1.3, which indicates that this film could be considered an antireflection coating. Finally, the spectral properties of Lumogen coatings were characterized by transmission, absorption, photoluminescence emission, and excitation spectra. It is showed that these coatings were transmitted well in visible region (lambda > 470 nm), and emitted a yellowish green glow centered at approximately 523 nm together with a wide excitation spectrum field from 240 nm to 490 nm. The synthesis shows that Lumogen coatings match accurately with the detected spectrum of conventional silicon-based image sensors, which makes this kind of thin films an ultraviolet responsive coating for sensors.