Microlens arrays are used to increase the optical fill factor of charge coupled devices (CCDs) with a compromised aperture that resulted from metal shielding. These arrays focus light onto the photodiode surface rather than letting it fall on the device’s non-photosensitive areas where the imaging information the device has collected can make the light disappear.
Adding Microlens Arrays to CCD Photodiodes
Addig the arrays can increase the optical fill factor by up to three times. An increase in the fill factor increases the photosite’s sensitivity. Micro lens arrays offer a significant increase in the way interline-transfer CCD imaging arrays perform with lateral overflow drains and a sizeable amount of shielded pixel space. Typically, these devices experience decreased optical fill factors due to decreased active pixel area compared to the total pixel size. Adding microlens arrays can have the fill factor approaching 100%, depending on the manufacturing parameters.
Achieving a Numerical Aperture
The optical characteristics of a microscope or camera lens used for directing light to CCD determine how the cone of light reaching the microlens is organized. Also, the thickness of the polysilicon gate has a great influence on the ability to collect light by the photodiode placed beneath the gate structure. Fabricators of microlens arrays use reflow techniques on resist layers to achieve numerical aperture that ranges from 0.15 to 0.4 with short focal lengths and corresponding lens diameter of 20 to 800 microns. An array’s fill factors depend strongly on the manufacturing process used for making the array. Also, the process involves the use of glass microlenses with lower numerical aperture, which means fewer optical aberrations with longer focal lengths.
Possible Challenges Associated with Microlens During Fabrication
Although there are difficulties encountered with microlens, the increase in the sensitivity of devices that have these optical components in place far outweighs them. When it comes to microlens, one of the challenges to face is when light rays from the outer parts of a pixel are focused onto an adjacent lens, leading to misregistration. Moreover, if the detector pixel size reaches the microlens’ diffraction limit, the pixels become overfilled, resulting in inaccurate measurements. When photodiodes reduce in size, the issues associated with producing quality microlens increase. A higher-quality microlensis required to produce images on such arrays; however, an issue like spherical aberration occurs. With the addition of microlenses to CCDs, the number of processing steps increases and the lens’ uniformity is a variable that can lead to issues during fabrication.