CMOS process enhancement for high precision narrow linewidth applications

During the last years CMOS technologies found widespread use in the development and fabrication of optical sensors and imagers [1][5]. However, during the development of CMOS photo diodes for special applications requiring the detection of radiation with a small spectral linewidth various aspects have to be considered that are negligible for photo diodes employed in common imaging applications. One very important aspect is the influence of interference effects due to the dielectric stack that covers the photo diodes in every CMOS process. This results in a dramatic modulation of the electronic signal as a function of the wavelength, stack thickness and spectral width of the impinging radiation. The uncertainties introduced by these oscillations disfavor standard CMOS imaging solutions for small spectral width applications. In common imaging applications with a larger spectral linewidth these modulations are not visible due to the convolution of the spectral sensitivity with the incoming spectral profile. This paper describes a CMOS process addition that can be applied to photo diodes to significantly reduce the sensitivity modulations by etching the dielectric stack in photo active areas. Using this approach the sensitivity modulations due to interference effects have been nearly eliminated for wavelengths above 300 nm. This enables the use of standard CMOS processes for spectroscopy or special imaging applications like laser illumination.