Smartphone Camera Advances Made Possible by mems|cam™ Technologies
Traditional autofocus camera modules employ voice coil motors to move the lens module along the optical axis of the camera. This technology – originally patented in 1874 – has reached a point of diminishing returns, where further reduction in size or cost creates unacceptable performance compromises.
Mechanical actuators based on silicon MEMS have fundamentally different mechanical and electrical characteristics, as well as the advantage of modern manufacturing processes. This makes them highly suitable for next-generation miniature autofocus cameras, providing greater autofocus speed, dramatically reduced power consumption, and higher precision. The mems|cam™ module from DigitalOptics now brings these benefits to smartphone cameras.
Mechanical actuators based on silicon MEMS have fundamentally different mechanical and electrical characteristics, as well as the advantage of modern manufacturing processes. This makes them highly suitable for next-generation miniature autofocus cameras, providing greater autofocus speed, dramatically reduced power consumption, and higher precision. The mems|cam™ module from DigitalOptics now brings these benefits to smartphone cameras.
More than two billion miniature cameras are manufactured annually for the cell phone and tablet markets. Of these, about 40% use autofocus (AF). An AF camera uses an actuator to move one or more lenses along the optical axis, while an algorithm calculates a figure of merit for the image sharpness for that lens location. The focus is then changed accordingly by moving the lens, and a new figure of merit is obtained. By repeating this procedure, the best focus for the scene can be obtained.
As the mobile phone has evolved, a very thin form factor has become a paramount design consideration. Compressing an 8-13 MPixel AF camera into a package 5 mm high while still producing high fidelity images is extremely challenging, but it is now a necessity in the modern smartphone. This white paper highlights the speed, power, and performance advantages of replacing the voice coil motor (VCM) with a MEMS actuator. DOC’s mems|cam delivers these benefits with MEMS autofocus in smartphone cameras.
The Voice Coil Motor
The incumbent actuator technology for miniature AF cameras is the voice coil motor. VCMs are named as such because they are based on the principles of attraction and repulsion between magnets to generate sound from electricity . The technology was first patented in 1874. See More The Voice Coil Motor.
MEMS Actuators
Silicon MEMS technology is able to integrate all three parts of a linear actuator into a single component. As shown in Fig., these are a stage, to provide vertical movement, a spring to provide the restoring force, and an electrostatic comb drive to displace the stage. See More MEMS Actuators.
Characteristics of MEMS AF Cameras
DOC’s mems|cam integrates a MEMS actuator and novel optics (Fig.) to deliver the benefits discussed below.
Faster and More Accurate Focus:
Consumers complain that miniature AF cameras with the incumbent VCM technology are slow to achieve focus. This prevents “capture-the-moment” photography. mems|cam modules are two to four times faster than VCM, thus enabling “instant” autofocus and support for 60 fps continuous focus video.
There are several reasons for the dramatic speed advantage offered by mems|cam:
1) Fast settling time: The mems|cam moves a single lens weighing a mere 3.5 mg, but a VCM must move the entire lens module holder, weighing about 45 mg. VCMs must also travel farther, approximately 250 μm from infinity to macro, compared to just 80 μm for MEMS. The silicon MEMS comb drive system has inherently less oscillation, typically <10 ms, compared to the metal springs and magnets of VCM.
2) Less hysteresis: mems|cam modules can also execute autofocus algorithms faster than VCM-based cameras due to improved positional accuracy and lack of hysteresis. The MEMS AF actuator is extremely accurate, with <1 μm of directional hysteresis. A VCM is far less predictable in its location due to directional hysteresis (typically about 10-20 μm), temperature dependencies, and coil resistance variation. This necessitates open-loop control with multiple adjustments before the focus is
correct. MEMS by contrast can operate closed-loop and more rapidly execute efficient AF algorithms.
3) Faster Algorithms: DOC’s patented FastFaceFocus™ technology accelerates autofocus when people are in the image by detecting a face and immediately focusing at that distance.
This combination of benefits leads to significantly faster autofocus speed. Figure 5 compares measurements of the VCM autofocus of leading smartphones with MEMS autofocus.
Lower Power Consumption and Cooler Operation: One of the main sources of power drain during image capture is operation of the VCM. Peak consumption can be as high as 200 mW, while a MEMS AF actuator, being electrostatically driven, requires only 1 mW. The power is needed only to partially charge or discharge a 200 pF capacitor. The capacitor is an air-gap device and therefore high quality with negligible loss. The camera is the single most power consuming function in current smartphones, so the shift from VCM to MEMS AF directly addresses one of the largest factors in battery life and thermal effects on the image sensor.
The excess power consumed by a VCM generates more heat, as shown in the thermal testing results in Fig. 6. Two problems are caused by this elevated temperature:
Image Sensor:
CMOS image sensors suffer from thermally induced problems including reduction in sensitivity, which is often wavelength dependent, an increase in background noise or “dark current” (doubling with every 6° of temperature increase), and degradation of micro lenses and color filters.
Optics:
Thermal expansion causes shift in the focal length of the lens module. This means that mechanical stops that set the infinity focus rest position might not always result in a focused image. Secondary effects include changes in the shape of the lens surfaces and shifts in the refractive indices of lens materials, both of which reduce image fidelity. The net effect is a 5-10% decrease in image quality for every 10° of temperature increase.
Better Precision:
In addition to the image quality benefits of a lower operating temperature, MEMS actuators also help lens tilt, one of the most challenging image quality issues for VCM cameras. VCMs have a typical dynamic tilt of >0.25°, causing defocus and a fall-off in image quality. In comparison, the dynamic tilt of MEMS is less than 0.1°. The 5X improvement in tilt translates to a 2X better corner image quality and a 3X larger “in-focus” window. Fig. 7 shows interferometer data of MEMS vs. VCM tilt.
Similarly, de-centering of a VCM, due to the floating action between the two planar springs, can be as much as +/-50 μm. This can give rise to vignetting and other optical defects. A MEMS actuator will be centered within +/-0.1 μm through its stroke, thus ensuring good optical fidelity throughout the focus range.
Smaller Z-Height and XY-Footprint:
The small size of a MEMS AF actuator allows it to fit inside the lens module. This contrasts with a VCM where the magnets, springs, coils, and housings must be mounted outside the lens module, adding to the XY footprint. Similarly, the reduced lens travel in a MEMS design (moving only the top lens 80 μm), allows the Z-height of a MEMS camera to be up to 170 μm shorter than comparable VCM designs, since VCMs move the entire lens barrel with 250 μm of travel. DOC’s proprietary flip chip packaging technology affords further reduction in Z-height (Fig. 8). The benefit can vary for specific designs, but a MEMS AF camera will always be smaller than an equivalent VCM AF camera.
Better Reliability and Longer Lifetime:
VCM springs are made of metal, which is a malleable material. As a result, the position and stiffness of the spring vary over time and use, and they are subject to fatigue. Even small mechanical or thermal shock loads can permanently change the rest position and the spring rate calibration of the VCM. This in turn leads to changes in lens barrel tilt, centering, and actuator threshold current. All of these variations affect the image quality and the time required to focus.
In marked contrast, the actuator at the heart of a mems|cam module uses silicon springs, which are ten times stronger than steel and are not malleable or subject to fatigue. As a result, the mechanical movement of the actuator will be the same after five years of use as it is the day it was made. This leads to stable image quality and consistently high-speed focus.
While VCMs are typically rated only for 300,000 cycles, MEMS actuators are rated for up to 10,000,000 cycles. With the shift to video for both making calls and image capture, the increased cycle life reliability is even more important.
Particle-Free Assembly:
Particles, if present and of sufficient size, can block light from reaching the imager, giving the appearance of defective pixels in the image. By design, a VCM AF camera has to be unsealed since there must be a clearance space between the lens holder and the yoke to allow for movement. This passageway permits particles in the air to reach the sensor. In a mems|cam module, only the first lens moves. This permits the static portion of the lens module to be sealed over the imager, preventing particles from reaching the imager.
Simple, Scalable, Solid-State Construction:
Fig. 9 shows the components of a disassembled VCM. As can be seen, it consists of many mechanical components, including springs, coils, magnets, housings, and guides. Assembling these parts requires many operations, all of which take time, cost money, and contribute to a lack of precision in the final assembly.
In comparison, a silicon MEMS actuator suitable for an AF camera requires only two components, the silicon actuator, which includes the integrated stage, springs and comb drive, and a housing (Fig. 10). This considerable reduction in piece parts simplifies assembly and improves reliability. Furthermore, the mechanical elements in a MEMS actuator are defined lithographically using semiconductor wafer processing techniques. This provides an aggressive path to support future cost reduction through wafer diameter increases and continual yield improvement, the cornerstones of progress in semiconductor manufacturing for decades.
Powered By
View comments