Slides of invited talk on ALD for MEMS at the AVS-ALD conference ALD 2009 Monterey, California, USA --- Full reference: R. L. Puurunen, M. Blomberg, H. Kattelus, ALD layer in MEMS fabrication, 9th International Conference on Atomic Layer Deposition “ALD 2009”, Monterey, California, July 19-22, 2009. Invited talk.

1. ALD layers in MEMS fabrication
Riikka L. Puurunen,
Martti Blomberg, Hannu Kattelus
VTT Technical Research Centre of Finland

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Puurunen et al., ALD 2009, 20 Jul 2009
Outline
1. ALD for MEMS – why?
2. Relevant properties of ALD layers regarding MEMS
3. ALD as enabling techology,
VTT Case: tunable Fabry-Perot interferometer
4. Conclusion

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Puurunen et al., ALD 2009, 20 Jul 2009
Outline
1. ALD for MEMS – why?
2. Relevant properties of ALD layers regarding MEMS
3. ALD as enabling techology,
VTT Case: tunable Fabry-Perot interferometer
4. Conclusion

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Puurunen et al., ALD 2009, 20 Jul 2009
What is MEMS?
typical dimension: micrometers
• Micro ElectroMechanical Systems
mechanically
moving parts
electrical circuits for
control/detection
sensing
actuation
• small size, lightweight
• low power consumption
• low price

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Puurunen et al., ALD 2009, 20 Jul 2009
VTT’s MEMS activities – Micronova, Espoo
• Design, prototyping, and production of MEMS
• Clean room area ~1850 m2, personnel ~100
Poly-Si MEMS
(Ultrasonic transducer)
SOI MEMS
(Silicon microresonator)
Integrated MEMS
(Altimeter)
Amorphous metal MEMS
(RF varactor)

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Typical steps in MEMS fabrication
-- similar to those in IC industry
Materials Temp. Conformality
SiO2
>800°C excellent
Si, SiO2
, Si3
N4
, … >400°C good
SiO2
, Si3
N4
, … >150°C poor
Mo, Al, Au, … ~50°C poor
photo resist ~100°C poor
• Material added by:
• Oxidation
• LPCVD
• PECVD
• Sputtering
• Spin coating
Cross section:
Capacitive Si microfone
• A low-temperature, conformal
material addition step is
missing ALD fills a gap

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Puurunen et al., ALD 2009, 20 Jul 2009
ALD brings in new materials
Example:
• Typical MEMS / IC insulators: SiO2, Si3N4
• Typical ALD insulators: Al2O3, TiO2, Ta2O5
New materials, new properties; for example:
• Aluminium oxide is practically inert in fluorine plasmas
• Titanium oxide is biocompatible, high-κ, high index
• Tantalum oxide is chemically resistant

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Puurunen et al., ALD 2009, 20 Jul 2009
Potential use of ALD in MEMS
• Electrically insulating conformal layers at low temperatures
• Etch masks, etch stop layers
• Conductive seed layers for plating
• Thermally conductive conformal layers
• Hydrophobic layers decrease of stiction
• Hermetic coatings
• Biocompatible coatings
• Closing of nanoscale pores
• Optical layers (reflective, anti-reflective, black absorbers)
• Layers reducing frictional wear
• …
First reports of ALD in MEMS year 2002 developing area

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Puurunen et al., ALD 2009, 20 Jul 2009
ALD equipment are now available for MEMS processing
• Previously ALD equipment vendors concentrated on:
• Display panel manufacturing tools; large glass substrates
• High-κ MOS gate on 200-300 mm wafers
• Dominant substrate in MEMS is 150 mm silicon wafer
tools now exist
• At VTT: Al2O3 and TiO2 (Picosun SUNALETM R-150 reactor)

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Puurunen et al., ALD 2009, 20 Jul 2009
Outline
1. ALD for MEMS – why?
2. Relevant properties of ALD layers regarding MEMS
3. ALD as enabling techology,
VTT Case: tunable Fabry-Perot interferometer
4. Conclusion

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Puurunen et al., ALD 2009, 20 Jul 2009
Mechanical properties
• Young’s modulus (elastic modulus), stress, Poisson ratio,
hardness, thermal expansion coefficient, …
• Little data for ALD films, properties may not be ”as expected”
• Most data for Al2O3 - Pioneering work by Tripp et al., 2006
• Missing, e.g.: Poisson ratio, trends with ALD temperature
100 150 200 250 300
0
200
400
600
Tripp et al. 2006
Herrmann et al. 2007
Tapily et al. 2008
amorph. alumina (King, 1988)
Young'smodulus(GPa)
ALD temperature (°C)
100 150 200 250 300
0
5
10
15
Hardness(GPa)
ALD temperature (°C)
Tripp et al. 2006
Herrmann et al. 2007
Tapily et al. 2008

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Mechanical properties
stress critical for surface micromachining
• For free-standing membranes,
bending stiffness dictated by (tensile)
stress
• Little data for ALD films published
• Tuning the stress unexplored
Puurunen et al., ALD 2007
50 100 150 200 250 300
0
200
400
600
Tensilestress(MPa)
ALD temperature (°C)
Tripp et al. 2006
Puurunen et al., ALD 2007
Al2O3 ALD temperature (°C)
50 100 150 200 250 300
0
200
400
600
0°
90°
Tensilestress(MPa)
TiO2 ALD temperature (°C)

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Electrical properties
• Dielectric constant, breakdown voltage, leakage current,
resistivity…
• Plenty of data to be found in literature
• Properties characterized in function of ALD process
temperature would be ideal
• Info needed also after ”medium” post-ALD heating (e.g. 500°C)
• For RFMEMS, also GHz measurements relevant
• Relevant thicknesses for MEMS ~2-200 nm thicker films than in
down-scaling CMOS
• Measurements on Si not necessary, MIM structures sufficient
• Presence of semiconducting Si often complicates the system,
• e.g. Al2O3: high-quality insulator also in the as-deposited state,
see poster by Puurunen & Kattelus

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Optical properties
• Refractive index, absorption coefficient (UV-visible-IR)
• Influence of ALD process temperature (and other parameters)
• Influence of thermal treatments
• Optical properties of ALD layers often differ from ”standard” materials
• Refractive index also needed for film thickness determination
0.8
1.0
1.2
1.4
1.6
1.8
2.0
0 2000 4000 6000
Number of cycles at 300°C
Refractiveindex
at633nm
Plasmos
FilmTek
1.6
1.7
1.8
1.9
2.0
2.1
2.2
0 200 400 600 800 1000
Wavelength (nm)
Refractiveindexn
300°C IMEC (Boher, Bender)
300°C VTT Filmtek
Nanospec "alumina"
Nanospec "aluminum oxide"

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Chemical properties
• Resistance to wet & dry chemical environments
Knowledge on passivation possibilities
patterning of layers
note the smaller (1/10x) y scale!
0
10
20
30
0 100 200 300
TiO2 ALD temperature (°C)
Etchrate(nm/min
0
100
200
300
0 100 200 300
Al2O3 ALD temperature (°C)
Etchrate(nm/min
BHF, 21°C
PSG etch, 21°C
poly etch, 21°C
SC-1, 65°C
HF dip, 21°C

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ALD process characteristics desired for
prototyping / small-scale MEMS production
• Uniform films on 150 mm wafers
• Processes stable in the time scale of years
• Flexible processes (temp., substrates, …)
• Simple use (many users)
• Commercially available reactants
• Contamination issues in control
/2006 25/05/2007 25/05/2008
30
40
50
60
AO-N300, 500 cyc
Al2O3thickness(nm)
Schematic process diversity in one ALD reactor:
IC industryUniversity research MEMS prototyping

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Changes in the properties during high-temperature steps
• In MEMS, after ALD often steps at higher temperatures (e.g. 500°C)
• Change in ALD layer properties during the high-temperature steps?
500 600 700 800 900 1000
1.65
1.66
1.67
1.68
1.69
1.70
1.71
1.72
Refractive index at 633 nm
Thickness
Al2O3 annealing temperature (°C)
Refractiveindexat633nm
84
86
88
90
92
94
96
Thickness(nm)
200 400 600 800 1000
200
400
600
800
1000
1200
Tensilestress(MPa)
Al2O3 annealing temperature (°C)

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Puurunen et al., ALD 2009, 20 Jul 2009
Outline
1. ALD for MEMS – why?
2. Relevant properties of ALD layers regarding MEMS
3. ALD as enabling techology,
VTT Case: tunable Fabry-Perot interferometer
4. Conclusion

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Puurunen et al., ALD 2009, 20 Jul 2009
Tunable Fabry-Perot MEMS interferometers,
basics
Fused Silica
• A gap separated by reflective
surfaces / mirrors, with the width
of the gap adjustable
• Transmission of light at specific
wavelenghts only (constructive
interference), width of gap
specifies transmission wavelength
• Length scale:
• Gap λ/2 (or n*λ/2, n = 1,2…)
• Bragg mirrors of uneven
number (3, 5, …) of high-low-
index materials: thickness
λ/(4n)
Filter chips, 3x3 mm2

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Puurunen et al., ALD 2009, 20 Jul 2009
VTT work on Fabry-Perot interferometers (FPIs)
Earlier work: infrared-FPIs for
CO2-measurement (in production)
• Si-substrate
• poly-Si and SiO2 mirrors
• SiO2 oxide as sacrificial layer,
release with HF
• Pass-band around CO2-absorbtion
4.3µm
FPI Thermopile
CO2
FPI Thermopile
CO2
New target: Visible light FPI (380-750 nm) new fabrication process needed
• Poly-Si difficult to scale to thickness <100 nm (pinholes) other high-
index material ALD Al2O3, TiO2
• ALD layers incompatible with HF polymer as a sacrificial layer
• Si no more transparent glass substrate

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Visible FPI process flow
VTT Monolithic Spectrometer
a) bottom mirror
(Al2O3 and TiO2)
b-c) bottom
electrodes
d) sacrificial
polymer layer and
top electrodes
e) top mirror and
patterning
f) release in O2-
plasma
a)
c)
e)
b)
d)
f)

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Transmission spectra of FPI
Large tuning range
• AC control enable tuning
range of the FPI gap in
excess of 60% from
1300 nm 500 nm.
Narrow peak, high
transmission
• The FWHM of the 4th
order transmission is 5.4
nm with maximum
transmission about 67%
at 500 nm.
2nd order 4th order 3rd order
0
0,1
0,2
0,3
0,4
0,5
0,6
0,7
0,8
450 475 500 525 550
Wavelength (nm)
Transmission
Vpp=0V
Vpp=17V
Vpp=21V
Vpp=26V
Vpp=27V
Vpp=23V
Vpp=29V

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Puurunen et al., ALD 2009, 20 Jul 2009
p+
+
nn- Si
VTT Monolithic spectrometer compared to state-of-the-art
VTT monolithic
spectrometer
Boehringer
Ingelheim
microParts GmbH
Micro-spectrometer
Horiba Jobin-
Yvon
Micro-
spectrometer
Dimensions TO-5, diam.=9.2
mm, Height 4.2 mm
54 mm x 32 mm x 9.5
mm
34.5 mm x 13.5
mm x 9.5 mm
Spectral range (220)350 – 1100 nm 350 – 850 nm 380 – 760 nm
Spectral resolution @ FWHM 2 – 7 nm < 10 nm < 5 nm
Minimum Transmission at
full spectral range
> 70 % > 30 % > 30 %
Relative manufacturing cost 1.0 4.0 8.0
Data, thanks to: Heikki Saari, VTT

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Enabling role of ALD in the FPI design
• New process design based on sacrificial polymer and non-poly-Si
metals low-temperature mirrors needed
• Al2O3 and TiO2 in use at VTT Micronova cleanroom suitable for
mirror fabrication
• Refractive index of Al2O3 ~1.65, TiO2 ~2.4
proof of concept easy to realize
• Issues in the FPI process (UV-vis-IR):
• Significant tensile stress of ALD layers only UV, vis
• Slowness of ALD price
• ALD is a new technique in MEMS process integration
concerns with polymers that allow higher temperatures,
other CVD techniques (PECVD) compete with ALD

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Puurunen et al., ALD 2009, 20 Jul 2009
Outline
1. ALD for MEMS – why?
2. Relevant properties of ALD layers regarding MEMS
3. ALD as enabling techology,
VTT Case: tunable Fabry-Perot interferometer
4. Conclusion

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Puurunen et al., ALD 2009, 20 Jul 2009
Conclusion
• The use of ALD in MEMS still largely unexplored
• Useful data is missing from the literature,
• Especially: Mechanical properties, stress
• Also: Optical properties, chemical properties
• Real first commercial break-through of
ALD in MEMS yet to be seen
p+
+
nn- Si

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Acknowledgements
• Funding, ALD development, Tekes projects:
ALDKOMP, ALEBOND, Nanoramems
• Funding, FPI: VTT Strategic Research
• ALD development and FPI:
Teija Häkkinen, Jukka Lappeteläinen, Meeri Partanen, Anna
Rissanen, Kimmo Solehmainen, Heikki Saari, Sari Sirviö, Mari
Ylönen

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VTT creates business from technology