Thin Film Packaging for MEMS

Transcription

Thin Film Packaging For MEMS
SEMI Networking Day Italy - 20/09/2012
D. Saint-Patrice
CEA, LETI, MINATEC
damien.saint-patrice@cea.fr
+33 (0) 4 38 78 06 39
Outline
 MEMS requirements
 Thin Film Packaging an attractive solution
 What’s the TFP state of the art?
 TFP and low pressure specifications
 TFP vs wafer bonding: comparative cost analysis
 Conclusion
SEMI Networking Day Italy
20/09/2012
© CEA. All rights reserved
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Leti at a Glance
Founded in 1967 as part of CEA
CEO Dr. Laurent Malier
1,700 researchers
190 PhD students + 34 post PhD
with 70 foreign students (30%)
250 M€ budget
~ 40M€ CapEx
Over 1,700 patents
265 generated in 2010
40% under license
37 start-ups
& 265 industrial partners
200 and 300mm Si capabities
8,000 m² clean rooms
Continuous operation
Léti/Minatec at Grenoble (F)
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Problematic of MEMS packaging
• Particles
PHYSICAL / CHEMICAL IN (Sensor)
• Humidity
MECHANICAL
PROTECTION
• Vibration
• Light
• Mechanical shock
• Gas
• Thermal stress
• Pressure
• EM waves…
• Acceleration
• Electromagnetic field…
ELECTRICAL IN
MOVING PARTS
VACUUM GAS FLUID
ELECTRICAL OUT
PHYSICAL OUT (Actuator)
Require specific & complex packages => Important overcost
Objective: To manage specificity at the wafer level (collective process)
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Thin Film Packaging an attractive solution
 Many advantages compare to other packaging techniques:
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Reduced area required for packaging
Very low thickness – tens of µm
Contact pad opening easy – no need for TSV
Process with standard equipments
No need for bonding tool
63 % saving
No need for second wafer
33 % saving
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Thin Film Packaging categories
 Two main types of TFP depending on the sacrificial layers:
 Mineral sacrificial material (most of the time the same as MEMS)
RF switches and accelerometers TFP with SiO 2 sacrificial layer [1]
Resonator TFP with SiO2 sacrificial layer [2]
 Organic sacrificial material
RF variable capacitor with 8 µm
polymer sacrificial layer [3]
RF BAW filter with polymer sacrificial
layer [4]
RF BAW resonator with polymer?
sacrificial layer [5]
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LETI Thin Film Packaging process flow
 LETI mainly focus their developments with organic sacrificial layer to:
 Minimize the thermal budget of the TFP process (<350 °C)
 Be compliant with topology on MEMS substrate
 Be less aggressive during the release process
 Schematic process flow:
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Polymer sacrificial layer deposit & patterning
Sacrificial layer curing
Cap deposition
Release hole etching
Sacrificial layer
Cap release
Sensitive part
Cap sealing
MEMS wafer
 But what is the state of the art of this technology?
 Back-end compatibility, mechanical structure, reliability, outgassing…
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TFP and Back-end compatibility
 Electrical performances on BAW resonator [6]
BAW process
UBM process
TFP
bumping
grinding
Wafer sawing
Plaque P02 - Filtre D20_top
cellule C9
1.95
2.05
2.15
2.25
0
-2
P ack0
-4
P ack1
P ack3
Sdd21 (dB)
-6
P ack4
-8
-10
-12
-14
-16
-18
-20
F (GHz)
 Same electrical performances before and after TFP + back-end
processes
 But reinforcement layer mandatory to be compatible with
overmolding (100 bars / 200 °C)
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TFP and Overmolding compatibility
 LETI developed different reinforcement processes
SiO2 cap
Resonator + TFP
+ Cu 23µm
Reinforcement
layer
Cap reinforcement with metal [6]
Cap reinforcement with epoxy [6]
Sacrificial release
hole
Cap reinforcement with localized
metal [7]
 BAW resonator electrical performances not affected by 100 bars
and 185 °C overmolding [6]
Molding epoxy
Cu 23µm
200µm
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TFP and low pressure specifications
 Impact of package miniaturization
1.0E-01
Top surface of the substrate
m)
30
1
th (µ
45
401
Leng
600
1.0E-02
201
Sealing material
Cap material (SiO2 or SiN)
1.0E+00
1000
 Outgassing from materials inside TFP cavity (major factor)
 µleak
Surface
 f ( Length; Height )
Volume
 Permeation
1.0E+01
800
 Pressure increasing can come from:
Surface / Volume
(µm-1)
1.0E+02
15
0
Height
(µm)
- Passivation (SiO2 or SiN)
- Metal lines (Au, Al…)
Sensitive part
MEMS wafer
Sensitive part of the device
 Possible schemes to reach low pressure TFP cavities:
 Optimized materials and outgassing process before sealing the cap
 Implement getter materials
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 Optimized materials and outgassing process before sealing the
cap
 Materials outgassing properties, one of the key parameter [8]
1.E+02
Outgassing (mbar.cm)
1.E+01
Sample
Al/TiN
SiN
SiO2 TEOS
SiO2 HDP
1.E+00
1.E-01
1.E-02
1.E-03
0
50
100
150
200
250
300
350
Thermal treatments (°C/30')
400
450
500
 Outgassing properties depend on: material itself, deposition process, thermal and
process history...
 Outgassing is critical above their thermal deposition temperature (mostly for PECVD
materials)
 SiN is a good outgassing barrier
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 Optimized materials and outgassing process before sealing the
cap
 Chemical composition outgassing is another key parameter  RGA analysis
mandatory
Mass spectrometer
Sealed MEMS put in a High vacuum
chamber
Open cavity in the chamber
Résiduel_[statique_série5-dynam ique_série1]
6.0E-11
Analyze gas present
in the chamber
Casse ampoule
5.0E-11
Best outgassing process can now be
defined
(max temperature, temperature ramp up,
process time…)
I (A)
4.0E-11
3.0E-11
2.0E-11
1.0E-11
0.0E+00
5
15
25
am u
35
45
2 benches available at Leti
(Resolution N2 0.3 -Ar 0.02pmoles)
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 Implement getter materials
Sensitive part
MEMS wafer
Getter material is able to
pump residual gases [13]
 But getter effect depends on gas present inside the cavity (better the outgassing,
better the getter)
 Tunable activation temperature (to fit with sealing process)
Getter properties
Activation temperature °C
500
450
AuSi
400
AuSn
350
Anodic
SDB
300
250
200
0,15
0,2
0,25
3
N2 sorbing capacity mbar.cm /cm²
Leti catalog of getters
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Polymer
 TFP sealing layer(s)
 Polymer sealing for device not working under vacuum (i.e BAW)
 Metal(s) sealing is the most used sealing layer(s) for vacuum specs
Hole
Cap
Release hole sealed
3.0 µm sealing layer
Al sealing materials [8]
Metallic sealing materials [9], [10]
Sealing layers
SiO2 sealing
Cap
Ti/Cu sealing materials [7]
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TFP pressure performances summary
 Pressure measurement: Q factor monitoring
 Few results published !
Thermal budget
(oC)
Molecular
Glass frit
Wafer bonding techniques
Anodic
AuSi
AuSn
[9]
TFP
950 oC
[12]
[10]
[8]
[11]
[5]
[2] [6]
TFP without getter
TFP with getter
10-4
10-3
10-2
10-1
1
101
102
103
Pressure
(mbar)
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Comparative cost analysis
 Comparing between Wafer Level Packaging methods
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Thin film packaging (standard) ~ 30 steps
Thin film packaging (with reinforcement) ~ 40 steps
Si cap packaging (polymer bonding) ~ 40 steps
Si cap packaging (with TSV) ~ 60 steps
 Evaluations based on a cost model taking into account:
 Global process (die area, yields…)
 Process flows (equipments CoO, operator time, consumables,…)
 Clean room environment (HU, depreciation, footprint, production capacities…)
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Comparative cost analysis : die area
Si cap packaging
Thin film packaging
1000µm X 700µm
1100µm X 800µm
Layout based on different constraints for the same design rules:
• Same electrical contacts geometries (120*120 µm)
excepted for Cap with TSV
• Same distances between cutting line and electrodes
• Sealing strip 60µm thick
About 20 % gain in die area achieved
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Comparative cost analysis : results
TSV DRIE
Align /
Bonding
Align /
Bonding
Thick Cu
ECD
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Conclusion
 Today Thin Film Packaging:
 Low cost packaging technique
 Clearly compatible with device working at near atmospheric pressure
(depending on the atmosphere specs)
 Overmolding compatible
 Vacuum packaging demonstrated until 10-3 mbar [9]
 Trends:
 Optimize TFP to be compatible with vacuum devices (gyro, accelero…)
 Develop TFP with controlled atmosphere
 Perform reliability testing
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References
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[1] V. Rajaraman, "Robust Wafer-Level Thin-Film Encapsulation of Microstructures using Low Stress PECVD Silicon
Carbide," Micro Electro Mechanical Systems, 2009. MEMS 2009. IEEE 22nd International Conference on , 25-29
Jan. 2009
[2] Bin Guo et al, "Poly-SiGe-Based MEMS Thin-Film Encapsulation," Microelectromechanical Systems, vol.21,
no.1, pp.110-120, Feb. 2012
[3] M. Endo et al, “Low Cost and Reliable Packaging Technology for Stacked MCP with MEMS and Control IC
Chips”, Internationale Symposium on Microelectronics, 2009
[4] J.L Pornin et al, "Wafer Level Thin Film Encapsulation for BAW RF MEMS," Electronic Components and
Technology Conference, 2007. ECTC '07. Proceedings. 57th , vol., no., pp.605-609, May 29 2007-June 1 2007
[5] K. Seetharaman et al, “A Robust Thin Film Wafer-Level Packaging Approach for MEMS Devices”, IMAPS, 2010
[6] J.L Pornin et al, "Low cost Thin Film packaging for MEMS over molded," Electronic System-Integration
Technology Conference (ESTC), 2010 3rd , vol., no., pp.1-4, 13-16 Sept. 2010
[7] J.L Pornin et al, “Cost effective thin film packaging for wide area MEMS”, ECTC, 2012
[8] D. Saint-Patrice et al, "Low temperature sealing process for vacuum MEMS encapsulation," Electronic
Components and Technology Conference (ECTC), 2012 IEEE 62nd , pp.97-101, May 29 2012-June 1 2012
[9] G. Dumont et al, “Pixel Level Packaging for uncooled Infrared Focal Plane Array”, MINAPAD, 2011
[10] Y. Naito et al, "High-Q torsional mode Si triangular beam resonators encapsulated using SiGe thin film,"
Electron Devices Meeting (IEDM), 2010 IEEE International , vol., no., pp.7.1.1-7.1.4, 6-8 Dec. 2010
[11] Dumont G. et al, “Innovative on-chip packaging applied to uncooled IRFPA”, Infrared technology and
Applications XXXIV, proc. of SPIE Vol. 6940, 69401Y (2008)
[12] Candler et al, "Long-Term and Accelerated Life Testing of a Novel Single-Wafer Vacuum Encapsulation for
MEMS Resonators," Microelectromechanical Systems, Journal of , vol.15, no.6, pp.1446-1456, Dec. 2006
[13] Lionel Tenchine et al, “NEG thin films for under controlled atmosphere MEMS packaging”, Sensors and
Actuators A: Physical, Volume 172, Issue 1, December 2011, Pages 233-239
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Thin Film Packaging for MEMS

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