Wafer-Level Lens Alignment
- Tuesday, 26 January 2010
New Factors to Consider
We need to look at the other important attributes for cell phone camera modules that are working against picture quality: low-cost, low-light ability (ISO speed), low current consumption, and small size.
Low-cost lenses exhibit significant variation in their optical characteristics within a batch. These variations cannot be determined by the physical appearance of the lens, but from the optical characteristics of each individual lens. The traditional assembly process of screwing the lens assembly into a housing to a set focal length does not take this variation in optical qualities into account. This misalignment did not matter so much when VGA sensors were predominant, as any misalignment of the lens was lost in the low image quality caused by the low pixel density. That is no longer true with multi-megapixel sensors.
This challenge will continue to be exacerbated as new miniaturized cameras are developed. While the CMOS sensor has traditionally been silicon wafer-based, breakthroughs in lens manufacturing technologies have enabled a new miniaturized camera — the waferlevel camera — to be developed. Waferlevel lenses are manufactured in large numbers in a wafer format. Wafers of different lenses are then stacked together to produce a complete lens which can be mounted directly onto the CMOS sensor. If the CMOS sensor also contains camera logic as well as the light sensitive pixels, then the result is a multimegapixel camera-on-a-chip within a small cube of just a few millimeters.
Wafer-level cameras have some significant advantages over traditional camera modules, which are made with barrel lenses. They are significantly smaller, which allows for a thinner format cell phone, they can be more cost-effective at around $1 to $3 for a VGA to 2 megapixel module, and they can be assembled as standard surface- mount components rather than manually assembled using ribbons and connectors, further adding to cost-savings.
Despite all these advantages, the variations in the lens manufacturing process across the lens wafer produces variations from lens to lens, and aligning and stacking these lenses on top of each other can result in significant centration misalignment and stackup tolerances. The very fact that these lenses are mounted directly onto the CMOS chip requires a very short focal length that is extremely sensitive to minor misalignments in assembly. It often requires an alignment of the lens to the chip that is significantly tighter than the tolerance of the lens itself.
These developments mean that alignment is critical to within microns of the focal plane. Yet any high-precision assembly process that relies on the physical dimensions of the sensor or lens produces poor performing cameras. This is only accentuated the smaller the camera system. While wafer-level lenses exhibit good optical qualities, their manufacturing process produces an even more significant variation from lens to lens. The sensor is high pixel density, the form factor is very small, the components are cost-effective. The problem is that the resulting misalignment produces a very poor picture quality. A wider lens (for better low light performance) just accentuates the problem.
Improvements in Camera Module Assembly
At this point, it is the assembly process that is the gating factor for improving the camera image. The alignment and assembly of these wafer-level cameras needs to be optimized for every single sensor-lens pair, depending on its particular optical characteristics, to within about a micron of its optimal focal length. But don’t forget that the sensor is not a point – it is a plane. So the assembly process must align the lens using 5 degrees of freedom (3 linear and 2 rotational axes) to the optical characteristics of the lens, not the physical characteristics of the lens. This is done by either measuring the path of a laser reflecting off the sensor and through the lens, or by powering up the sensor and optimizing the image during the assembly process. The latter is called active camera module alignment. Once the optimal position is found, the lens must be affixed in that position relative to the sensor.
This method of alignment gives an enormous boost to the performance of the camera module (Figure 3), because the weakest link in the camera system has now been so significantly improved that the performance enhancements of greater pixel density and lens innovations can start to shine through.