Atomic Layer Deposited Alumina for Micromachined Resonators

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Atomic Layer Deposited Alumina for Micromachined
Resonators

• Y. J. Chang, K. Cobry, and V. M. Bright
• University of Colorado, Department of Mechanical
  Engineering, Boulder, USA
• Refer from MEMS 2008, Tucson, AZ, USA

Team Cheng-Yi Lin (???) Yen-Po Lin
(???) Date November 11, 2008
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Outline

• Abstract
• Introduction
• Resonator
• Atomic Layer Deposition
• Fabrication
• Measurement of Micor-resonators
• Displacement
• Frequency
• Conclusion

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Abstract

• Atomic layer deposited (ALD) aluminum (Al2O3) as
  the material for micro-resonator
• A pinned-pinned beam model with axial stress of
  250MPa is used
• Higher modes of the resonator were observed

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Introduction (I) Resonator

• Device with a vibratory nature response
• Application Sensors of pressure, mass, or force
  with high resolution
• Material Silicon, Aluminum nitride
• Method Chemical vapor deposition (CVD), Atomic
  layer deposition (ALD)

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Atomic Layer Deposition Alumina
Advantage

• Used on structure with high ratio and irregular
  geometries
• High quality
• Pinhole free
• Uniform
• Deposited at low temperature

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Deposited Method

• (A)
• (B)
• 1. Precursor is introduced into the viscous flow
  reactor
• 2. A precursor (trimethyl aluminum) is purged
  form the chamber with nitrogen
• 3. B precursor (water) reacts with all of the
  available sites on the A layer that is introduced
• 4. B precursor is purged from the chamber with
  nitrogen
• 5. Again this step until the film is of the
  desired thickness

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• AB binary surface reaction sequence for ALD

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Fabrication Process of ALD-based Resonator
Cr 5nm
Al2O3 85nm
Coat 85 nm Al2O3 on Silicon
Deposit 5 nm Cr by e-beam evaporation
Spin coat PR AZP 4210
Etch Al203 in 5 HF (37s)
Remove PR AZP 4210
Release Al2O3 structure by isotropic silicon etch
with SF6 plasma
Pattern and develop PR AZP 4210
Etch Cr in CR-7(13s)
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Fabrication Process
550 um
5nm
85nm
Figure 4 SEM image of ALD Al2O3 micro-resonators
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Measurement (I) Displacement

• Clamped-clamped model
• Pinned-pinned model
• Distributed transverse electrostatic force

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Experimental Results

• Theoretical and experimental results of
  displacement vs. voltage of ALD Al2O3
  micro-resonator

• Measured and theoretical calculated profiles of
  the micro-resonator with different applied
  voltages

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Measurement (II) Frequency

• Resonant frequency

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Experimental Results

• Theoretical and experimental data of resonance
  modes

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Conclusion

• Micromachined ALD Al2O3 resonators had been
  demonstrated
• High quality thin film
• Nano-scale resonator
• Displacement and resonant frequencies have been
  calculated by pinned-pinned beam model
• Axial stress in the resonators of 250MPa is
  determined by fitting the experiment data

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Reference

• 1 G. Stemme, Resonant silicon sensors, J.
  Micromech.Microeng., vol. 1, pp.113-125, 1991.
• 2 A.N. Cleland, M. Pophristic, and I. Ferguson,
  Single-crystal aluminum nitride
  nanomechanical resonators, Appl. Phys. Lett.,
  vol. 79, pp.2070-2072
• 2001.
• 3 N. D. Hoivik, J.W. Elam, R.J. Linderman, V.M.
  Bright,S.M. George, and Y.C. Lee, Atomic layer
  deposited protective coatings for
  micro-electromechanical systems, Sensors and
  Actuators A, vol. 103, pp. 100-108
• 2003.
• 4 M. K. Tripp, C. Stampfer, D.C. Miller, T.
  Helbling, C.F. Herrmann, C. Hierold, K. Gall,
  S.M. George, and V.M. Bright, The mechanical
  properties of atomic layer deposited alumina for
  use in micro- and nano-electromechanical
  systems, Sensors and Actuators A, vol.130-131,
  pp. 419-429, 2006.
• 5 M. K. Tripp, C.F. Herrmann, S.M. George, and
  V.M. Bright, Ultra-thin multilayer nanomembranes
  for short wavelength deformable optics, in Proc.
  of MEMS04, Maastricht, The Netherlands, Jan.
  25-29, 2004, pp. 77-80.
• 6 B. Hälg, On a nonvolatile memory cell based
  on micro-electro-mechanics, in Proc. of MEMS90,
  Napa Valley, California, Feb. 11-14, 1990,
  pp.172-176.
• 7 S. D. Senturia, Microsystem Design, Kluwer
  Academic Publishers, Massachusetts, USA, 2001.
• 8 R. D. Blevins, Formulas for Natural Frequency
  and Mode Shape, Van Nostrand Reinhold Co., New
  York, USA, 1979. 390

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Thank you for your attention
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Q A Time
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Comparison of ALD and CVD

• ALD
• Highly reactive precursors
• Precursors react separately on the substrate
• Precursors must not decompose at process
  temperature
• Uniformity ensured by the saturation mechanism
• Thickness control by counting the number of
  reaction cycles
• Surplus precursor dosing acceptable

• CVD
• Less reactive precursors
• Precursors react at the same time on the
  substrate
• Precursors can decompose at process temperature
• Uniformity requires uniform flux of reactant and
  temperature
• Thickness control by precise process control and
  monitoring
• Precursor dosing important

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• Need to compensate the bending when stress is
  present

• The form of the polynomial

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• Types of Support

Free
Fixed
Pinned
Pinned on Rollers