AFM Data Storage Systems

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AFM Data Storage Systems

• Jia-Yang Juang
• Prof. D. Bogy
• Computer Mechanics Lab
• University of California, Berkeley
• Dec. 5, 2001

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Outline

• Introduction
• AFM Cantilevers
• Cantilevers for Writing
• Cantilevers for Readback
• Multiple tip Arrays
• Further Considerations

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Introduction

• Limit of magnetic recording (60-100 Gb/in2)
  expected superparamagnetic limit
• Ultrahigh areal density 400-500 Gb/in2
• Terabit capacity
• Small form factor miniaturized low-power
• High data rate 10 Mb/s for readback
  100 kb/s for writing

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Introduction (cont.)
Comparison between AFM and conventional ones
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Introduction (cont.)
Research Groups on AFM Data Storage Systems

• IBM Zurich Research Laboratory, Switzerland.
  (Vettiger, Despont, Durig, Binnig, etc.) -
  Millipede 32X32 AFM array
• Collaboration between Stanford Chui, Kenny,
  Goodson, Quate, etc. IBM (Almaden Research
  Center) Mamin, etc.

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AFM Cantilevers

• Thermomechanical data storage systems
• Components media (polymer), tip with sensors,
  moving scheme
• Digital information is represented as data pits
  on a polymer
• The sharp tip contacts with the polymer by a
  weak force 0.1 uN

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Cantilevers for Writing

• Force heating at tip
• Writing speed is limited by thermal time
  constant0.34um X 51um X 40um0.6us1um X 100um X
  16um1us

Heavy doping
Light doping
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Cantilevers for Readback

• Piezoresistive sensing (Chui, Kenny, Mamin,
  etc.)
• Thermomechanical sensing (IBM Zurich
  Research Lab)

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Piezoresistive Sensing

• Piezoresistivity deflection ?? change in
  resistance
• Cantilever deflection is measured as the tip
  rides over pits
• Data rates depend on the resonant frequency
  0.34um X 10um ? f2.6MHz ?10Mbit/s (expected)

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Thermomechanical Sensing
Changes in temperature are monitored while the
cantilever is scanned over data bits, providing a
means of detecting the bits
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Writing Reading Animation
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Erasing Rewriting

• Erasing data by heating the polymer to 150C
  for a few seconds
• Erase larger storage areas (no bit-level erase)
• Erasing, rewriting, and bit stability are
  topics of ongoing research.

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Moving Schemes

• Stylus-based scheme (phonograph) (Chui,
  Kenny, Mamin, Vettiger, etc.)

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Moving Schemes (cont.)

• Scanning approaching scheme (Millipede,
  IBM Zurich Research Lab)

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Multiple Tip Arrays
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One Cantilever Cell

• Tip radius 20nm
• legs 50um X 10um X 0.5um
• platform 10um X 5um X 0.5um
• stiffness 1 N/m
• resonant frequency 200KHz
• thermal time constant 1us

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SEM of the cantilever array section
Photograph of fabricated chip (14 mm X 7 mm)
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Tips Characteristics

• Tip radius 20nm
• Tip height 1.7um
• Tip height homogeneity in an array 50nm(0.5)

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SEM of Millipede Writing Results
Storage fields
Magnified views. Bit indentations/separations(100
-200Gb/in2)
Thermomechanical read-back
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Further Considerations

• Overall system reliability, including bit
  stability, tip and medium wear,
  erasing/rewriting.
• Limits of data rate (S/N ratio), areal density,
  array and cantilever size.
• CMOS integration.
• Optimization of write/read multiplexing scheme.
• Array-chip tracking

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Fabrication

• Pattern oxide for tip
• Etch tip
• Pattern piezoresistor
• Pattern silicon nitride
• Pattern aluminum
• Pattern cantilever

Silicon micromachining steps for cantilevers with
tips and resistors
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Fabrication (cont.)

• Deep trench etch through wafer
• Cantilever release

Backside silicon micromachining steps for
suspending cantilevers