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“The problem with that,” says Bereziuk, “is that chip-on-board techniques require a clean room. If a particle of dust drops on a pixel….. Second, as the pixel size continues to decrease the number of pixels increases and again, you can damage it if particles fall on the pixels. Now, the trend is toward packaging—if dust falls on a package sensor it can be cleaned because it’s a glass surface.”
Tessera licenses its technology to Nemotek and to Q-Technologies (Kunshan City, Jiangsu Province, China). It includes OptiML Wafer-Level Camera (WLC), as well as Shellcase Wafer-Level Chip Scale Packaging (WLCSP). (Tessera bought wafer-level-packaging expert Shellcase Ltd. (Jerusalem); Digital Optics Corp. (Charlotte, N.C.), a specialist in wafer-level micro-optics technology; and Eyesquad (Tel Aviv, Israel), which has autofocus and optical-zoom expertise. Shellcase provides wafer-level packaging for both surface-mount and wirebond cameras. Digital Optics applies microelectronics techniques to optics, allowing production of well-aligned lenses on wafers. Eyesquad’s digital focus technology becomes crucial when wafer-level cameras need 2-megapixel resolution or better. Its autofocus and digital zoom offer depth-of-field for higher-resolution cameras without using moving parts.)
Tessera’s OptiML WLC technology promises scalability with pixel size as well as reflow compatibility. The process of aligning and bonding the lenses at the wafer level eliminates the need for manual focusing. The optical elements use reflow-compatible materials and are available in both wirebond and (ball grid array) BGA formats. The imager package must have a BGA interface on its rear surface. A BGA is a standard component interface in the semiconductor assembly industry and comprises an array of solder spheres of closely dimensioned size in specified locations. Having a BGA interface allows the imager to be placed together with other components on a common printed circuit board (PCB) and all to be attached and interconnected by exposure to a single reflow soldering cycle. This process is known as surface mount assembly. When BGA is used, the optical element can be directly placed on a board.
The process works in three segments, according to a white paper by Hongtao Han and Keith Main of Tessera’s Charlotte, N.C. facility: fabrication and singulation of wafer-level optics; wafer-level chip scale packaging and singulation of image sensors; and integration of wafer-level optics with packaged image sensors.
The fabrication step begins with UV replication of micro-optics on a wafer scale. Depending upon the optical design, micro-optics can be fabricated on both sides of glass wafers. Next comes wafer bonding, optical testing and singulation (conversion of wafers into separate dies) of the optics wafer stack.