James Dekker, ack. Jaakko Saarilahti, Jyrki Kiihimäki, Hannu Kattelus

VTT TECHNICAL RESEARCH CENTRE OF FINLAND
OUTLINE
• Introduction
• Ultrasonic transducers from polysilicon
• The Plug-Up process
• SOI Resonators
• variations
• Amorphous metals

VTT TECHNICAL RESEARCH CENTRE OF FINLAND
INTRODUCTION
• Different micromachining technologies:
• Surface Micromachining
• polysilicon and metal layers
• oxide as sacrificial layer
• Example:Acoustic emission sensor
• Bulk Micromachining
• anisotropic etching (TMAH)
• SOI-based Micromachining
• ICP etching
• Buried oxide sacrificial layer
• Example :Resonators
Both surface and SOI processes benefit from a novel release etch procedure used at VTT

VTT TECHNICAL RESEARCH CENTRE OF FINLAND
CAPACITIVE MICROMACHINED ULTRASONIC TRANSUCER
(CMUT)
• A device for detecting ultrasonic pressure waves (6 to 13 MHz)
• NDT and ultrasonic imaging
• Surface micromachined using polysilicon
• Fully functional 500 element
CMUT matrix has been demonstrated (1 mm2)
• A Novel method for etching of the sacrificial layer has been used.

VTT TECHNICAL RESEARCH CENTRE OF FINLAND
CMUT PROCESS
BEGIN
• Process begins with
LTO+poly + 600 nm
TEOS depositions
• Deposition and patterning of nitride
• Deposition of porous poly-Si
• Cavity formed by HF etch and SC drying, then sealed with poly Si
• More depositions and patterning to get final structure
5. Silicon nitride deposition 200 nm
6. Silicon nitride patterning
7. Polysilicon deposition
8. Cavity etching and sublimation
9. Polysilicon deposition 300 nm
13. Al –metallization patterning
END
Al -metal
Polysilicon
Silicon nitride
Polysilicon
Silicon oxide

VTT TECHNICAL RESEARCH CENTRE OF FINLAND
RELEASE ETCHING OF THE CMUT MEMBRANE
Removing the sacrificial oxide with HF
Al -metal
Polysilicon
Silicon nitride
Polysilicon
Silicon oxide
D = 40 - 60 um

VTT TECHNICAL RESEARCH CENTRE OF FINLAND
CHARACTERIZATION OF RESONANCE
Resonance C_15_A3
Osc Level=1 V pp DC Bias = 40 V
2.00E+03
1.60E+03
1.20E+03
0.00
-20.00
-40.00
8.00E+02
4.00E+02
-60.00
-80.00
0.00E+00 -100.00
6.50E+06 7.00E+06 7.50E+06 8.00E+06 8.50E+06
Frequency [Hz]
1.50E+07
1.30E+07
1.10E+07
9.00E+06
• Q= 100
• PULL-IN VOLTAGE ~40-200 V
Resonance frequency
C15 Gap = 600 nm
A -type
D -type
E -type
7.00E+06
5.00E+06
400000 500000 600000 700000
Effective mass 1/SQRT(m)
800000

VTT TECHNICAL RESEARCH CENTRE OF FINLAND
LAME AND BAW RESONATORS from SOI
• Resonators for RF applications require high Q values with low power consumption.
• Low phase noise (Quartz resonators are ~-150 dBc/Hz)
• Bulk acoustic mode offers excellent characteristics compared to flexural mode
• 12 MHz BAW
• 13 MHz Lame
• gap=1 um (mask) by ICP
~400 um

VTT TECHNICAL RESEARCH CENTRE OF FINLAND
RESONATOR PROCESSING
• All MEMS processing is CMOS compatible
• 5-10 um SOI, 1 um BOX
• pattern metal
• ICP etch resonator and gaps
• HF release etch
• Supercritical drying
• Non-IC processing (esp. metals) done at back-end frequency
111 kHz
12 MHz type
Cantilever
BAM
Q (vacuum)
1.6 MHz bridge (c-c) 30000
14 MHz bridge (c-c) 1500
3000
5
2x10

VTT TECHNICAL RESEARCH CENTRE OF FINLAND
CHARACTERIZATION
3.0
2.5
2.0
1.5
1.0
100 Hz
0.5
11.7479
11.7480
11.7481
11.7482
11.7483
f (MHz)
6
11.7 MHz BAW
4
2
0
-2
-4 Q=180 000
-6
11.7479
11.7480
11.7481
11.7482
11.7483
f (MHz)
• Measurement of S-parameters and resonance frequencies
• Phase noise (-115 dBc/Hz at 1 kHz offset)
6
13.1 MHz Lame
4
2
0
-2
-4
Q>100 000
-6
13.0952
13.0953
13.0954
13.0955
13.0956
f (MHz)

VTT TECHNICAL RESEARCH CENTRE OF FINLAND
ALTERNATIVE SOI-PROCESS FOR RELEASING LARGE
STRUCTURES
Pattern and etch release holes, strip, line with poly
Nitride
Etch cavity in HF and SC Dry
Thin
Poly-Si
Fill with poly, etchback, repattern, etch gaps to release structure
Poly-Si • Gaps and release holes by
ICP etching
• Structure released by HF etch followed by SC drying
• suitable for small or rigid structures
Better yield for large structures
No holes in structure

VTT TECHNICAL RESEARCH CENTRE OF FINLAND
MEMS, Amorphous Metals, and IC integration
MEMS first?
IC first?
Metallurgy!
Topography!
Complexity!

VTT TECHNICAL RESEARCH CENTRE OF FINLAND
DC
Reactive co-Sputtering of Mo-Si-N
C:
Mo
34
Si
20
N
41
5.3 g/cm 3
0.75 m
W cm
(O
5
)
N
B:
Mo
19
Si
26
N
49
4.2 g/cm 3
4.8 m
W cm
(O
6
)
Shutter
Ar
Wafers
N
2 Target
MoN
Mo
2
N

-Mo-N
Mo
A
C
Mo
3
Si Mo
5
Si
3
B
MoSi
2
Si
3
N
4
Si

VTT TECHNICAL RESEARCH CENTRE OF FINLAND
Thermal Stability
200 nm Mo-Si-N layers
80
60
RT
800
900
1000
500
600
700
40 as-deposited
20
1100
Dark-Field
0
20 40 60 80 100 120 140 160 180 200 o
Temperature ( C)
1000C / 1min in Ar
Conductive
MoN or MoSi precipitates ?

VTT TECHNICAL RESEARCH CENTRE OF FINLAND
Microelectromechanical test device: variable capacitor
• Sputter MoSiN onto resist
• Dice
• O2 plasma “release etch”
Photoresist
O + O +
O +
O +
O +
3.5
3.0
2.5
2.0
1.5
1.0
0.5
0.0
0 2 4 6
Varactor CV Curve
8 10
Voltage (V)
12 14 16 18 20

VTT TECHNICAL RESEARCH CENTRE OF FINLAND
Conclusions
- Amorphous metallic alloys are interesting alternatives for silicon in fabricating MEMS devices
- Polymeric materials can be used for sacrificial layers
- Stress is more uniform and controllable than for polycrystalline metals
- Mo-Si-N is an IC-compatible material candidate
- Low deposition temperature (down to room temperature)
- High thermal stability

VTT TECHNICAL RESEARCH CENTRE OF FINLAND
Summary
- Surface and SOI-based micromachining are dominant processes at VTT
- New release technology facilitates the fabrication of complex structures
- Amorphous metallic alloys are interesting alternatives for silicon in fabricating MEMS devices