Spectral filtering for image sensors

In order to design and realize multiple wavelength bandpass filters for image sensors, we have investigated optical multilayer dielectric filters. The filter design is based on a chirped Bragg grating which is approximated by a stack of two alternatively deposited dielectric materials. Multi-layer stacks have been simulated with the rigorous chain-matrix method. The reflection and transmission coefficients of the stacks have been obtained by the multiplication of the respective 2 x 2 characteristic matrices of each layer. Moreover the period and chirping of the discrete chirped Bragg gratings have been optimized to match the blue spectral response. The filter band has been designed to transmit the 400-500nm wavelengths and to reflect the other visible spectrum wavelengths. Single layers of Silicon oxide and Silicon nitride have been deposited using a PECVD (Plasma-Enhanced Chemical Vapor Deposition) process. The characterization of these layers has been made by ellipsometry, and has led to a modification of the nitride deposition process to improve its optical characteristics. The complete stack has been deposited on a quartz substrate, and the reflectance and the transmittance have been measured. The next step is to improve the deposition process.