SAN FRANCISCO As image sensor resolution increases and pixel sizes decrease, the sensor production yields become more sensitive to particulates material particles that end up dropping onto the surface of the sensor.
A novel glass packaging scheme unveiled at the Semicon show here by Tessera Inc. (San Jose, Calif.) promises close to a 40 percent yield improvement for high-resolution image sensors mounted using chip-on-board schemes for camera modules.
COB mounting accounts for about 60 percent of all cell phone camera modules and thus this approach can have a significant cost impact said Mitch Reifel, manager of business development and licensing at Tessera. The packaging scheme, dubbed Shellcase CF, is the first offshoot of the glass sealing technology that Tessera purchased from Israel-based Shellcase Technology in late 2005.
The higher-resolution sensors (typically 2-Mpixels and higher) use smaller pixel areas and thus particles that may not have had an impact on low-resolution sensors with larger pixel areas, may actually cover entire pixels on the high-resolution sensors, said Reifel.
Furthermore, the surface of the imaging sensor is typically covered with millions of color microlenses and the texture of the material used for these lenses is such that particles like to adhere to the surface stated Reifel.
If these particles cannot be removed, they cause permanent image defects and render the module unusable. "Tessera's approach seals the sensor with a glass cap before particles have a chance to settle onto the chip surface. The glass surface also provides a non-stick environment so that particles can easily be removed before the final assembly is sealed by a camera lens", said Reifel. "This same approach can also be applied to MEMS devices and other optical components and is lower in cost than chip-scale packaging approaches."
"By placing a small gap between the glass surface and the sensor surface, a particle falling on the glass would also be out of the focal plane of the sensor array and thus have less impact on image quality, explained Reifel. "We estimate that about 40 percent of the high-resolution sensors that might be affected by particles can be recovered and that would drive up module yields and lower costs to a point where the extra cost of the glass bonding pays for itself" said Reifel.
To keep the sealing cost low, Tessera starts with a thin optical glass wafer into which "pockets" are created into each region where an imaging sensor would be covered. The glass wafer is then bonded to the wafer of imaging chips.
And then the glass is removed to expose the bond pads and scribe lines. Before the wafer is diced up, it systems can test each sensor since the bond pads are accessible, thus permitting vendors to weed out defective sensors before units are shipped to the module assembly lines.
After testing the sealed chips are singulated and sent off for mounting on the camera module boards. One additional benefit--since the sensors are protected, a lower-class clean room environment can be used, thus further lowering the overall production cost.