MEMS - Solving Auto-Focus for the Cell Phone Camera Market

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					MEMS: Solving
Auto-Focus for the
Cell Phone Camera
Roman Gutierrez
November 2010

    •   Introduction
    •   Passive Auto-Focus Types
    •   Auto-Focus Modifications
    •   Moving Lens Barrel
    •   Moving Single Lens
    •   MEMS Actuator Principle
    •   Auto-Focus Integration
    •   Particle Effects
    •   Summary

    Introduction to Auto-Focus

    • Auto-Focus (AF) automatically adjusts the optical system
      in a camera to ensure that the picture is sharp independent
      of the distance to the object
       • Active AF systems measure distance to the subject using a range
         finder, and subsequently adjust the optical system for correct focus
       • Passive AF systems determine focus position by analyzing the
         image while the optical system is adjusted for correct focus
    • Cell phone cameras use passive AF due to advantages
      in size and cost

    Passive Auto-Focus Types

    • Auto-focus using modification of an optical element
    • Auto-focus using motion of an optical element
       • Moving lens barrel
       • Moving single lens

                                 Image      Image
                       Lens      Sensor   Processing


                       Driver                AF
                       Circuit            Algorithm

    Auto-Focus Using Modification of an Optical Element

    •   Simple integration into the camera
    •   Spherical surface for tunable lens creates aberrations
    •   Increased z-height for module
    •   Hermetic sealing of liquids: environmental and reliability

                                                   Example: Liquid lens from Varioptic

    Auto-Focus Using Modification of an Optical Element

              Infinity                   10cm

    Optical Element Modification Auto-Focus Issues

    • Additional light loss from tunable optical element
    • Additional aberration from tunable optical element
    • Change of tunable optical element must include aspheric
      terms to avoid degradation in resolution

          10cm moving barrel           10cm using liquid lens

    Moving Lens Barrel of Auto-Focus

             Infinity                  10cm

    Moving Lens Barrel - Auto-Focus Example

    • Currently used in most cell phone AF cameras
    • Motion control is not precise
       • Reduced image quality due to tilt
       • Hysteresis and lack of repeatability
    • Uses electromagnetic actuator
      to apply force
       • Electromagnetic crosstalk with other
         components in the phone
       • High power and heat generation

                                                Example: Voice Coil Motor (VCM)

     Moving Lens Barrel – Auto-Focus Issues

     • Tilt of lens barrel must be precisely controlled
     • Lens barrel must be precisely adjusted along optical
       axis to focus
     • Lateral misalignments cause distortion and relative
       illumination problems

     Moving Single Lens in Auto-Focus

     • MEMS (Micro-Electro Mechanical Systems) is the only
       technology to move single lens’ for AF
     • Smallest size AF camera available (same as fixed focus)
     • Extremely low power (1/200 of voice coil motor)
     • Fast and precise
     • Requires precise alignment between moving lens and other

     Moving Single Lens in Auto-Focus

                           A (single lens movement)                                                                    14:08:56

                                                                                                                                           B (Lens barrel movement)
                                                                         DIFFRACTION LIMIT                                                                                              DIFFRACTION LIMIT       WAVELENGTH   WEIGHT
                         1/3.2"-5M-1.75u-F2.8                            AXIS
                                                                                                 WAVELENGTH   WEIGHT                    1/3.2"-5M-1.75u-F2.8                            AXIS                     650.0 NM     107
                                                                                                  650.0 NM     107
                         -4P                                         T
                                                                         0.6 FIELD ( 20.88 O )    610.0 NM     503                      -4P                                         T
                                                                                                                                                                                        0.6 FIELD ( 20.61 O )    610.0 NM     503
                             DIFFRACTION MTF                         T   0.8 FIELD ( 26.97 O )    555.0 NM    1000                          DIFFRACTION MTF                         T   0.8 FIELD ( 26.83 )
                                                                                                                                                                                                          O      555.0 NM    1000
                                                                     R                                                                                                              R
                                                                                                  510.0 NM     503                                                                  T
                                                                                                                                                                                                                 510.0 NM     503
                                               14-Dec-09             T   1.0 FIELD ( 32.28 O )                                                                14-Dec-09             R
                                                                                                                                                                                        1.0 FIELD ( 32.66 O )    470.0 NM      91
                                                                     R                            470.0 NM      91
                                                                                             DEFOCUSING 0.00000                                                                                               DEFOCUSING 0.00000
                   1.0                                                                                                            1.0

                   0.9                                                                                                            0.9

                   0.8                                                                                                            0.8

                   0.7                                                                                                            0.7

              M                                                                                                              M
              O 0.6                                                                                                          O 0.6
              D                                                                                                              D
              U                                                                                                              U
              L 0.5                                                                                                          L 0.5
              A                                                                                                              A
              T                                                                                                              T
              I                                                                                                              I
              O 0.4                                                                                                          O 0.4
              N                                                                                                              N
                   0.3                                                                                                            0.3

                   0.2                                                                                                            0.2

                   0.1                                                                                                            0.1

                                      40              80             120                   160                 200                                   40              80             120                   160                 200
                                                SPATIAL FREQUENCY (CYCLES/MM)                                                                                  SPATIAL FREQUENCY (CYCLES/MM)

     MEMS Actuator Principle

     • MEMS are fabricated on silicon wafers using large,
       established semiconductor foundries
     • Tessera’s MEMS actuator is based on a comb drive
        • Comb-drives are capacitive actuators that utilize electrostatic forces
          that act between two electrically conductive combs

        • The combs are arranged so they never touch
        • When a voltage is applied, it causes the combs to be drawn
          together, causing movement
        • The force (and movement) is proportional to the change in
          capacitance (and voltage applied) between the two combs

     MEMS-Based Auto-Focus Integration Into a Cell Phone
     Tessera          Lens Vendor          Integrator           OEM

     MEMS to           Attach moving       Electro-          System
     actuate single    lens                mechanical        integration
     lens for AF                           design
                       Complete lens                         AF algorithm
     Packaged with     barrel assembly     Select a combo    (can be
     plastic snubs                         driver (ADI,      provided by
                       Terminate AF        Maxim, STM) for   Tessera)
     Supplied with     and shutter         AF and shutter
     or without a      contacts on flex
     shutter blade                         Actively align
                       Attach EMI shield   lens barrel to
                       MTF testing
                                           SFR testing

     MEMS Single Lens Auto-Focus Advantages
     • Use single lens motion inside of lens barrel
         •   Enables lowest ‘z’ height
         •   Larger TTL for optical design
         •   No added z height to add shutter
         •   Faster AF due to reduced range of motion
             and lower mass
     • No lens barrel motion with respect to imager
         •   Reduces x-y size
         •   Reduces number of parts
         •   Improves alignment of optics to imager
         •   Reduces particles on imager window
     • Use MEMS electrostatic actuation integrated with MEMS motion control
         •   Improves lens alignment and movement precision
         •   Reduces size
         •   Reduces power consumption
         •   Reduces actuator assembly complexity and cost
         •   Integrated capacitive position sensing available

     Comparison of MEMS and Voice Coil Motor

     • MEMS low power, speed and reliability uniquely enable
       continuous AF in video applications
                                              MEMS              VCM            Piezo-motor    Liquid Lens         Liquid Crystal

            Actuator size (mm)          7x7x0.15         8.5x8.5x5        8.5x8.5x5          7.75 (circle) x2   4.5x4.5x0.5

                                                         8.5x8.5x6        8.5x8.5x6          8.5x8.5x8          8.5x8.5x7
                                        8.5x8.5x6        w/o shutter      w/o shutter        w/o shutter        w/o shutter
            Module size (mm)
                                        with shutter     10x10x8.5        10x10x8.5          10x10x10           10x10x9
                                                         with shutter     with shutter       with shutter       with shutter

            Supports 1/3” optical
                                        Yes              Yes              Yes                Yes                No

            Imager resolution           8 MP             8 MP             8 MP               3 MP               3 MP

            Driver size (# of pins)     7                7                18                 10                 15

            Peak power                  0.5 mW           250 mW           80 mW              8 mW               100 mW

                                                                          Needs position
            Repeatability               1 µm             10 µm                               N/A                N/A

                                                                          Needs position
            Hysteresis                  3 µm             20 µm                               Yes                Yes

            Speed                       5 ms             30 ms            30 ms              200 ms             500 ms

            Reliability cycles          10 million       100 thousand     100 thousand       1 million          1 million

            Reflow compatible           Yes              No               No                 No                 No

                                                         Image                                                  Requires
                                                                          Motion control     Oil and water
            Reliability environmental   No degradation   degradation                                            temperature
                                                                          degradation        separation
                                                         and hysteresis                                         control

            Acoustic noise              No               No               Yes                No                 No

     Effects of Particles on Imaging Systems
     • Dust particle located at (xd, yd) casts a shadow on focal plane
       array as shown below
     • Dust particle located at the entrance pupil casts shadow over
       entire focal plane array
     • Distance between entrance pupil and particle plane determines
       contrast and size of shadow on the focal plane array – CLOSER
       to entrance pupil is LESS sensitive to particles
                        Particle   Entrance      Focal
                         plane       pupil       plane

     Effects of Particles Comparison
     • Dust particles on the single lens moving AF likely end up either
       on L1 or L2 surfaces, about 100 micrometers away from the
       entrance pupil
     • Dust particles on the moving barrel AF likely end up on the
       imager window about 500 micrometers from the image plane
     • The shadow’s contrast depends on the particle’s size and the
       distance from the imager and is given by the table below for the
       two types of AF

                                  Auto-Focus Part Effects of Particles
             Particle size   Contrast for barrel moving AF   Contrast for single lens AF
             10 µm           0.3%                            0.003%

             30 µm           3%                              0.03%

             100 µm          30%                             0.3%

             300 µm          100%                            3%

     Sensitivity of Human Eye to Contrast
     • Contrast sensitivity for the human eye is a function of age and spatial
       frequency, as shown below
     • Lens barrel AF gets a double impact, as contrast is higher and spatial
       frequency is higher, so eye is more sensitive to contrast
     • Based on this data, particles sizes less than 300µm in size should be
       acceptable for single lens moving, whereas particles greater than about
       10µm would be a problem for lens barrel moving

     • Moving lens is best way to achieve Auto-Focus (AF)
     • VCM moves lens barrel to focus; MEMS moves single lens to
     • Advantages of single lens moving AF vs. barrel moving AF
        •   Includes a shutter without increasing size (TTL)
        •   Improved MTF, specially at close focus where AF is needed
        •   Better manufacturing yield
        •   Faster due to smaller travel and lower moving mass
        •   Reduced blemish
     • Advantages of MEMS vs. VCM
        •   Drastically reduced power consumption
        •   Smaller size
        •   Longer cycle life
        •   Continuous AF
        •   No interference with magnetic shutter
        •   Reduced assembly complexity

     Thank you.


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