Smartphone Camera Advances Made Possible by Mems Cam Technologies February 20131

8/13/2019 Smartphone Camera Advances Made Possible by Mems Cam Technologies February 20131

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Smartphone Camera Advances MadePossible by mems|cam Technologies

February 2013

www.doc.com


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Smartphone Camera Advances Made Possible by mems|cam Technologies

2013 DigitalOptics Corporation Inc. All rights reserved. 2

Abstract

Traditional autofocus camera modules employ voice coilmotors to move the lens module along the optical axis ofthe camera. This technologyoriginally patented in 1874has reached a point of diminishing returns, wherefurther reduction in size or cost creates unacceptable

performance compromises.

Mechanical actuators based on silicon MEMS havefundamentally different mechanical and electricalcharacteristics, as well as the advantage of modernmanufacturing processes. This makes them highly suitablefor next-generation miniature autofocus cameras, providinggreater autofocus speed, dramatically reduced powerconsumption, and higher precision. The mems|cammodule from DigitalOptics now brings these benefits tosmartphone cameras.

mems|cam autofocus module


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Table of Contents

Introduction ................................................................................................................. 4

The Voice Coil Motor................................................................................................... 5

MEMS Actuators .......................................................................................................... 7

Characteristics of MEMS AF Cameras...................................................................... 8

Faster and More Accurate Focus ....................................................................... 8Lower Power Consumption and Cooler Operation ............................................. 9Better Precision ................................................................................................ 10Smaller Z-Height and XY-Footprint ................................................................... 11Better Reliability and Longer Lifetime ............................................................... 11Particle-Free Assembly .................................................................................... 11Simple, Scalable, Solid-State Construction ...................................................... 12

The MEMS Revolution in Smartphones ................................................................... 13

MEMS vs. VCM Summary Table ............................................................................... 14


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Introduction

More than two billion miniature cameras are manufactured annually for the cell phoneand tablet markets. Of these, about 40% use autofocus (AF). An AF camera uses anactuator 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 isthen changed accordingly by moving the lens, and a new figure of merit is obtained. Byrepeating 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 paramountdesign consideration. Compressing an 8-13 MPixel AF camera into a package 5 mmhigh while still producing high fidelity images is extremely challenging, but it is now anecessity in the modern smartphone. This white paper highlights the speed, power, andperformance advantages of replacing the voice coil motor (VCM) with a MEMS actuator.DOCs mems|cam delivers these benefits with MEMS autofocus in smartphonecameras.


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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 andrepulsion between magnets to generate sound from electricity (see Fig. 1). The

technology was first patented in 1874.

Fig . 1: A VCM is the incumbent technology used in a miniature camera to move a lens moduleand alter focus. The technology, shown in an 1888 patent by Alexander Graham Bell (left), is

not fundamentally different from the VCM in a HTC OneX+ smartphone (right).

Operation of a VCM involves passing current through the electromagnet (coil). Thiscreates a magnetic field that is repulsive with respect to the permanent magnets,causing the lens holder to move vertically away from the image sensor. The restoringforce is provided by springs. The rest position is infinity focus.

It is increasingly apparent that VCMs are compromising the performance of small AFcameras. These effects include:

VCMs suffer from hysteresis of stroke, typically around 8%. The resulting variabilityof position complicates and slows the autofocusing process sufficiently to preventrapid photography and tracking of focus distance changes during video capture.

VCMs consume excessive power, typically more than 100 mW peak. This demandfor current drains the battery quickly and generates heat that degrades opticalperformance and image quality.

VCMs suffer from lens tilt and de-centering, typically 0.25 and 50 m, respectively.The requirement for clearance between the lens module holder and the yoke permitsthis uncontrolled movement, and both tilt and de-centering vary with the orientationof the camera and the focus position. These effects decrease the image fidelity.


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The problem becomes more acute as image sensor resolution increases, pixeldimensions decrease, and f-number decreases.

VCMs are challenged to serve the demand for thinner handsets with better imagequality. Making a VCM smaller requires smaller coils, magnets and springs.Because magnetic force is proportional to volume, smaller coils and magnets require

more current to create enough force for actuation, making the power consumptionand heat problems worse. Furthermore, smaller springs are weaker, exacerbatinghysteresis of stroke, lens tilt, reliability, and de-centering issues.

VCM technology is over 100 years old. This means that there are limitedopportunities for further cost reduction, thus providing a commercial opportunity for acompeting technology that can deliver better performance at a suitable cost.


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MEMS Actuators

Silicon MEMS technology is able to integrate all three parts of a linear actuator into asingle component. As shown in Fig. 2, these are a stage, to provide vertical movement,a spring to provide the restoring force, and an electrostatic comb drive to displace the

stage.

Fig. 2: Silicon stage (left), spring (middle), and electrostatic comb drive (right).

A comb drive is a pair of electrically conductive structures arranged so the interdigitatedfingers never touch. When a DC voltage is applied, the resulting electrostatic chargedevelops an attractive force that causes the combs to be drawn together. By attachinga lens in the center, a silicon MEMS autofocus actuator (Fig. 3) can be created.

Using MEMS, the movement of the stage can be precisely controlled. This makes itpossible to employ a novel optical configuration where only the first lens is moved, whilethe remainder of the lens module is locked in an optimal position. Using this approach,excellent image quality is obtained over the entire focus range from 10 cm to infinity,and a number of other important benefits also result.

Fig. 3: A MEMS autofocus actuator holds the lens in the center and moves it along the opticalaxis when electrostatic charge produces vertical motion in a comb drive.


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Characteristics of MEMS AF Cameras

DOCs mems|cam integrates a MEMS actuator and novel optics (Fig. 4) to deliver thebenefits discussed below.

Fig. 4: An exploded view of a mems|cam module.

Faster and More Accurate Focus: Consumers complain that miniature AF cameraswith the incumbent VCM technology are slow to achieve focus. This prevents capture-the-momentphotography. mems|cam modules are two to four times faster than VCM,thus enabling instantautofocus 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. VCMsmust also travel farther, approximately 250 m from infinity to macro, compared to

just 80 m for MEMS. The silicon MEMS comb drive system has inherently less

oscillation, typically


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correct. MEMS by contrast can operate closed-loop and more rapidly executeefficient AF algorithms.

3) Faster Algorithms: DOCs patented FastFaceFocus technology acceleratesautofocus when people are in the image by detecting a face and immediatelyfocusing at that distance.

This combination of benefits leads to significantly faster autofocus speed. Figure 5compares measurements of the VCM autofocus of leading smartphones with MEMSautofocus.

Fig. 5: Measurements of autofocus speed show mems|cam to be significantly faster than VCMautofocus in four leading smartphones.

Lower Power Consumption and Cooler Operation: One of the main sources ofpower drain during image capture is operation of the VCM. Peak consumption can beas high as 200 mW, while a MEMS AF actuator, being electrostatically driven, requiresonly 1 mW. The power is needed only to partially charge or discharge a 200 pFcapacitor. The capacitor is an air-gap device and therefore high quality with negligibleloss. 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 inbattery 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 problemsincluding reduction in sensitivity, which is often wavelength dependent, an increasein background noise or dark current (doubling with every 6 of temperatureincrease), and degradation of micro lenses and color filters.

Optics: Thermal expansion causes shift in the focal length of the lens module. Thismeans that mechanical stops that set the infinity focus rest position might not always

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result in a focused image. Secondary effects include changes in the shape of thelens surfaces and shifts in the refractive indices of lens materials, both of whichreduce image fidelity. The net effect is a 5-10% decrease in image quality for every10 of temperature increase.

Fig. 6: Thermal testing of MEMS vs. VCM autofocus shows a significant temperature differencedue to the higher power of VCMs. The measurements were with focus at

35 cm in a 25C ambient environment.

Better Precision: In addition to the image quality benefits of a lower operatingtemperature, MEMS actuators also help lens tilt, one of the most challenging imagequality issues for VCM cameras. VCMs have a typical dynamic tilt of >0.25, causingdefocus and a fall-off in image quality. In comparison, the dynamic tilt of MEMS is lessthan 0.1. The 5X improvement in tilt translates to a 2X better corner image quality anda 3X larger in-focus window. Fig. 7 shows interferometer data of MEMS vs. VCM tilt.

Fig. 7: Interferometer data shows MEMS AF to have 80% less tilt than VCM.

Similarly, de-centering of a VCM, due to the floating action between the two planarsprings, can be as much as +/-50 m. This can give rise to vignetting and other opticaldefects. A MEMS actuator will be centered within +/-0.1 m through its stroke, thusensuring good optical fidelity throughout the focus range.


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Smaller Z-Height and XY-Footprint: The small size of a MEMS AF actuator allows itto 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 XYfootprint. Similarly, the reduced lens travel in a MEMS design (moving only the top lens

80 m), allows the Z-height of a MEMS camera to be up to 170 m shorter thancomparable VCM designs, since VCMs move the entire lens barrel with 250 m oftravel. DOCs 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 willalways be smaller than an equivalent VCM AF camera.

Fig. 8: Flip chip packaging enables low z-height camera modules.

Better Reliability and Longer Lifetime: VCM springs are made of metal, which is amalleable 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 loadscan 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 siliconsprings, which are ten times stronger than steel and are not malleable or subject tofatigue. As a result, the mechanical movement of the actuator will be the same afterfive years of use as it is the day it was made. This leads to stable image quality andconsistently high-speed focus.

While VCMs are typically rated only for 300,000 cycles, MEMS actuators are rated forup to 10,000,000 cycles. With the shift to video for both making calls and imagecapture, the increased cycle life reliability is even more important.

Particle-Free Assembly: Particles, if present and of sufficient size, can block light fromreaching 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 betweenthe lens holder and the yoke to allow for movement. This passageway permits particles


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in the air to reach the sensor. In a mems|cam module, only the first lens moves. Thispermits the static portion of the lens module to be sealed over the imager, preventingparticles 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 partsrequires many operations, all of which take time, cost money, and contribute to a lack ofprecision in the final assembly.

Fig. 9: A typical voice coil motor, such as this one from the HTC One X+, requires theassembly many components, including springs, coils, magnets, and carriers.

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 combdrive, and a housing (Fig. 10). This considerable reduction in piece parts simplifiesassembly and improves reliability. Furthermore, the mechanical elements in a MEMSactuator are defined lithographically using semiconductor wafer processing techniques.This provides an aggressive path to support future cost reduction through waferdiameter increases and continual yield improvement, the cornerstones of progress insemiconductor manufacturing for decades.

Fig . 10: A MEMS autofocus component consists of just two components, a silicon actuatorintegrating a stage, spring, and comb drives, plus a low-cost housing.


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The MEMS Revolution in Smartphones

MEMS devices have revolutionized smartphones by creating the functionality offormerly stand-alone devices in a miniaturized, low-cost, high-reliability format suitablefor mobile handsets. Functions now supported by MEMS include microphones,

gyroscopes, accelerometers, pressure sensors, and clocks (Fig. 11). DigitalOptics isnow providing mems|cam modules for smartphones, ushering in the next phase ofMEMS fast, low-power MEMS AF that lets users catch the moment with theirsmartphone cameras while easing worries of battery life and image sensor degradation.Table 1 summarizes the many advantages of mems|cam vs. VCM autofocus cameras.

Fig. 11: MEMS components have already enabled many electro-mechanical functions forsmartphones. MEMS autofocus is next.


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MEMS VCM Significance

SPEED

Settling time

Smartphone Camera Advances Made Possible by Mems Cam Technologies February 20131

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