Micro arrays are used for increasing the optical fill factor in charge-coupled devices (CCDs) like interline-transfer devices that deal with a decreased aperture as a result of metal shielding. These micro lens systems concentrate light onto the photodiode surface rather than letting it fall on non-photosensitive areas of the device.
Placement of Optical Lens on CCD Photodiodes
In a typical lenslet placement scheme, a tiny optical lens is strategically placed over the photodiode’s metal light shield and dye layer. The lenslet is developed in parallel arrays during the fabrication of the CCD or made out of a material like quartz and placed on the array surface during packaging. Every lenslet is a high-quality optical surface that has refractive elements, ranging in size from several hundred to about ten microns in diameter, depending on the application. The quality of the lens is good that micro lenses are equivalent to a regular single-element lens.
Increasing the Optical Fill Factor
Adding microlens arrays to CCD photodiodes can significantly increase the optical fill factor without the tiny optical components. Such an increase produces a corresponding increase in the photosite’s sensitivity. Microlens arrays offer a significant increase in the performance of interline-transfer CCD imaging arrays with lateral overflow drains and a huge amount of shielded pixel space. Typically, such devices experience decreased optical fill factors due to reduced active pixel area compared to the total pixel size.
Organizing the cone of light that reaches the microlens surface depends on the optical characteristics of the camera lens or microscope lens used for directing light to the CCD. The thickness of the polysilicon gate also influences the ability of the photodiode placed beneath the gate structure to collect light. The fabrication of microlens arrays involves the use of reflow techniques on resist layers meant to achieve numerical apertures which range from 0.15 to 0.4 with short focal lengths and 20 to 800 microns corresponding lens diameter. An array’s fill factor depends on the manufacturing process used for making the array. Lenses made of glass with lower numerical aperture are also used.
Issues Associated with Microlenses During Fabrication
While there are drawbacks faced with microlenses, the increased sensitivity of devices with optical elements far outweighs them. One of the main issues that can take place when focusing light rays from the outer parts of a pixel onto an adjacent lens is misregistration. Additionally, if the detector pixel size reaches the microlenses’ diffraction limit, the pixels become overfilled, resulting in inaccurate measurements. The addition of microlenses to CCDs will increase the number of steps to be taken in the process and the lens array’s uniformity is a variable that can usually lead to issues during fabrication.