Microactuator for High-Capacity Hard-Disk Drivesmaecourses.ucsd.edu/~pbandaru/mae268-sp09/Class...

Hitachi Confidential2004/4/6Copyright 2003 Hitachi Global Storage Technologies

Toshiki Hirano, San Jose Research Center

Microactuator for High-Capacity Hard-Disk Drives

Toshiki HiranoHitachi Global Storage Technologies

San Jose Research Center


2004/4/6Copyright 2003 Hitachi Global Storage Technologies

Disk Drives in the past(IBM, 60 GB storage, 1980-1989)

Today:400 GB Hitachi Deskstar 7K400



Recording Density GrowthRecording al

Production Year Ed Grochowski

60 70 80 90 100 1101E-3

1E-2

1E-1

1E+0

1E+1

1E+2

1E+3

1E+4

1E+5

1E+6

Are

alD

ensi

ty M

egab

its/in

2

HGST Disk Drive ProductsIndustry Lab DemosHGST Disk Drives w/AFCDemos w/AFC

HGST Areal Density Perspective

1st MR Head

1st GMR Head

2000 10

60% CGR

Ultrastar 146Z10

Deskstar 180GXP

100% CGR

Corsair

Deskstar 16GP

Travelstar 30GNMicrodrive II

1st AFC Media

Future Are DensityProgress

Travelstar 80GN

Perpendicular

Superparamagnetic effect

105

104

103

102

106

10

1

10-1

10-2

10-3

25% CGR

IBM RAMAC (First Hard Disk Drive)

1st Thin Film Head3375

35 Million XIncrease



Track Density and Linear Density

1E+1

1E+2

1E+3

1E+4

1E+5

1E+6

90 92 94 96 98 100 102 104

Availability Year

Areal Density, mbits/in2

Track, Areal, Linear Density Perspective

02 04

10 5

10 4

10 3

10 2

10

Linear Density, kbits/in

Track Density, tracks/in

2000

Ed Grochowski

Track Density CGR = 50%

Areal Density CGR = 100%

Linear Density CGR = 30%

Circles = Server productsSquares = Mobile products

Hard Disk Drive Products

106

Linear Density

Trac

k D

ensi

ty

100 kTPI(250 nm pitch)



Track Pitch Challenge

1000X

95 mm95 m

Data track width:Data track width:250 nm

250 micrometer(human hair diameter x 3)



Minimizing Tracking Error

• The position error between magnetic head and data track on disk (called Track Mis-Registration-TMR) must be ~10% of track pitch.– For 250 nm track pitch, acceptable TMR is about 25 nm.

• Two factors affecting TMR:– Position Disturbance– Disturbance correction by close-loop control (servo)

• First-order TMR model:– TMR = Position Disturbance / Disturbance Correction by servo



Position Disturbance Sources

• Majority of position disturbance is caused by air-flow induced by spinning disk.– Example: 84 mm diameter disk spinning at 10,000 RPM– Linear speed at disk edge = 44 m/Sec ~= 100 MPH !

• Today’s disk drive has:– Extremely high stiffness (Disk, Actuator).– Aerodynamically designed mechanical components.– Optimization such that the vibration node coincides magnetic head

location.• Example: Suspension beam

• However, there is usually a limit on how well one can reduce this disturbance.– Example: Disk can not be infinitely thick, mechanical tolerances.



Disturbance rejection by servo

• Disk drive has a close-loop control system (servo) that corrects position disturbance.

• Close-loop consists of:– Position error measured by head -> Controller generates input to

voice-coil-motor (VCM) actuator -> Position error is corrected.

ServoController Head Position

Voice-Coil Motor Drive Signal



Servo Bandwidth• Servo performance is measured by “servo bandwidth”.

– Servo bandwidth is defined as the frequency where the open-loop gain crosses 0 dB line.

– Higher bandwidth enables better disturbance correction.– Typical servo bandwidth of current HDD product is 1-1.5 kHz– Higher bandwidth is required for future HDDs.

Gain

G(s)+

-+Input

Disturbance

Output(Position)

Error Rejection= = Position

Disturbance

1

1+G(s)

G(s): Controller x ActuatorFrequency(Hz)

0 dB

Servo Bandwidth

G(s)

Bode Plot of G(s)(Open-loop transfer function)



Servo bandwidth limitation• Servo bandwidth of current HDD is usually limited by mechanical

resonances of head-positioning actuator (VCM, arm, and suspension).

Pivot Friction

1/s^2(40 dB/dec)



Dual-Stage Actuator

• 1st Generation (Moving Suspension)– Prototyped by HTI, NHK, Magnecom, Fujitsu ...– PZT controlled– Limitation by suspension dynamics

• 2nd Generation (Moving Slider)– TDK PZT based (fragile)– MEMS prototyping: IBM/Hitachi, Seagate... – Challenge for fabrication / assembly

• 3rd Generation (Moving Magnetic Head)– Concept phase: U-Tokyo/Hitachi ...– Complicated integration of MEMS & magnetic

element

HTI

TDK

Hitachi

U-tokyo/Hitachi



MEMS Microactuator Developed at Hitachi SJRC

• Rotary actuator• Electrostatically driven• Spring support (no friction)• Stroke: +- 1 mm• Actuator mass: 3-4 mg• Plated nickel structure• Output force: ~0.5 mN



Electrostatic Actuator• Electrostatic force:

Ng

hVF ×=2ε

• To have large force output:• large h• small g• high V• More N

• Process capability limits aspect ratio (h/g).

• High-Aspect-Ratio process was developed.



High-Aspect Ratio Process• High-Aspect Ratio (up

to 20:1, or 40 um thick, 2 um wide)

• Metal (Nickel) Structure

• Process Steps:– Thick polymer coating– Highly directional oxygen

plasma etching of polymer (electro-plating mold).

– Nickel Electroplating.

2 um

40 um



Microactuator Design Example• Push-Pull• Three connections (Drive+, Drive-, GND or DC Bias)

2.1mm

1.7m

m

Anchor to the substrate(Fixed)

Slider Binding Platform(Movable)

Electrostatic Actuator

Rotational Spring



Fabrication

Metal

Anchor

Structure(40 um thick)

Stud(10 um)

Top cover(10 um)

• Mostly made by typical IC processes, such as photo-lithography, film deposition and etching.

• Some non-IC but magnetic head standard processes, such as electroplating

• Batch fabricated. – ~1500 chips from 4" wafer.– ~7000 chips from 8" wafer.



Process Steps (Cross Section)

Lithography and 2nd Ni plating

Polymer Coating (40 um)

Insulator coatingMetal Patterning

Lithography and 3rd Ni plating/Au plating

2nd Insulator PatterningSacrificial Patterning

High Aspect RatioPolymer Etching (20:1)

Thick nickel plating (40 um)

3rd Insulator Patterning

Polymer/resist removalReleaseBack-side lapping



Microactuator Chip Example



Example of finished wafer

8 inch~7000 Chips / wafer



Microactuator Assembly with slider and suspension

• Additional lead wire required:– Current 4 wires -> 7 wires

• Additional assembly step required:– Current two parts -> three parts



Microactuator assembly into drive

• Three extra wires



Microactuator Performance• Frequency response (From input voltage to position)

– Microactuator vs. conventional VCM actuator

-60-40-20

020406080

100 1000 10000 100000

Frequency (Hz)

Gai

n (d

B)

VCM Microactuator

-180

-90

0

90

180

100 1000 10000 100000

Frequency (Hz)

Pha

se (d

egre

e)Microactuator Mass/spring mode. Damped by close-loop controller



Servo Experiment

• Parallel design– Low frequency signal goes to VCM, and high frequency signal goes

to microactuator.

MicroactuatorController

VCMController

Microactuator

VCM

++

-

Digital Signal Processor Hard disk drive

MicroactuatorDisplacement

VCMDisplacement

PositionReferencePosition

PositionError



Servo Experiment Results (1)• Open-loop transfer function

– 0 dB cross-over frequency (servo bandwidth) of 8 kHz by microactuator achieved, compare to ~1.8 kHz conventional VCM servo.

-60

-40

-20

0

20

40

100 1000 10000 100000

Frequency (Hz)

Gai

n (d

B)

Microactuator+VCM VCM only

-180

-90

0

90

180

100 1000 10000 100000

Frequency (Hz)

Pha

se (d

egre

e)

Microactuator0 dB@8 kHz

VCM0 [email protected] kHz



Servo Experiment Results (2)• Error rejection function

– With microactuator, error rejection is better up to 6 kHz.

-50

-40

-30

-20

-10

0

10

100 1000 10000 100000

Frequency (Hz)

Erro

r Rej

ectio

n (d

B)

Microactuator+VCM VCM only



Contact recording with microactuator

– Microactuator has small moving mass, thus more sensitive to any external force.

10 100 1000 10000 100000

Frequency (Hz)

1.0E-12

1.0E-11

1.0E-10

1.0E-9

1.0E-8

1.0E-7

1.0E-6

Disturbance (m)

VCM Milli/VCM Micro/VCM

Force Input: 0.7mN Normal Contact Force * 0.3 Friction Coefficient * Sin(15) SkewVCM: Iz=4E-6 kg*m2, fr=70, Q=5, L=50mm, Milli: Iz= 4.5E-9, fr=9kHz, Q=30, L=11, Micro: Iz=5.7E-13, fr=2kHz, Q=1, L=0.625mm

Off-track Disturbance by Contact Force

Contact Force

Skew

k1

c1

Iz1 k2 c2

Iz2

Other MEMS Application in Data Storage

"MILLIPEDE""MILLIPEDE"

x

z 2

z3

z1y

Nanotechnology Nanotechnology entering Data Storage entering Data Storage

Thermomechanical Read/Write on Thin Polymer Media

32 x 32 (1024) Cantilever Array Chip

Micromechanical x|y|z Scanner

=>most recent areal density demo 1Tbits/inch2 (single lever)

40 nm pitch

40 nm bit size

Zurich Research LaboratoryMicro- and Nanomechanics Group

Courtesy of Dr. Peter Vettiger, IBM Zurich Lab



Conclusions• Moving-slider microactuator for tracking servo was

investigated.• Microactuator was successfully fabricated and

assembled into drive.• Servo experiment was carried out, and 8 kHz servo

bandwidth was demonstrated.– Conventional VCM has ~1.8 KHz bandwidth.

• Compatibility with contact recording needs to be investigated.

AcknowledgementThis research was carried out with other collaborators, including Henry Yang, Matthew White, Fu-Ying Huang, Francis Lee, Mr. Harpreet Singh, Ms. Karen Scott and other members at Hitachi San Jose Research Center.

Microactuator for High-Capacity Hard-Disk Drivesmaecourses.ucsd.edu/~pbandaru/mae268-sp09/Class...

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