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

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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

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Leti at a Glance

Founded in 1967 as part of CEA

1,700 researchers

37 start-ups & 265 industrial partners

Over 1,700 patents

250 M€ budget

CEO Dr. Laurent Malier

190 PhD students + 34 post PhD with 70 foreign students (30%)

~ 40M€ CapEx

265 generated in 2010 40% under license

Léti/Minatec at Grenoble (F)

200 and 300mm Si capabities 8,000 m² clean rooms Continuous operation

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Problematic of MEMS packaging

MECHANICAL

PROTECTION

• Particles

• Humidity

• Vibration

• Mechanical shock

• Thermal stress

• EM waves…

ELECTRICAL IN ELECTRICAL OUT

PHYSICAL / CHEMICAL IN (Sensor)

• Light

• Gas

• Pressure

• Acceleration

• Electromagnetic field…

PHYSICAL OUT (Actuator)

VACUUM GAS FLUID MOVING PARTS

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: 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

No need for second wafer

33 % saving

63 % 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)

Organic sacrificial material Resonator TFP with SiO2 sacrificial layer [2] RF switches and accelerometers TFP with SiO2 sacrificial layer [1]

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: Polymer sacrificial layer deposit & patterning

Sacrificial layer curing

Cap deposition

Release hole etching

Cap release

Cap sealing

But what is the state of the art of this technology? Back-end compatibility, mechanical structure, reliability, outgassing…

MEMS wafer

Sacrificial layer

Sensitive part

| 7

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TFP and Back-end compatibility

Electrical performances on BAW resonator [6]

Same electrical performances before and after TFP + back-end processes

But reinforcement layer mandatory to be compatible with overmolding (100 bars / 200 °C)

TFP UBM process bumping grinding Wafer sawing BAW process

Plaque P02 - Filtre D20_top

cellule C9

-20

-18

-16

-14

-12

-10

-8

-6

-4

-2

0

1.95 2.05 2.15 2.25

F (GHz)

Sdd21 (dB

)

Pack0

Pack1

Pack3

Pack4

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TFP and Overmolding compatibility

LETI developed different reinforcement processes

BAW resonator electrical performances not affected by 100 bars and 185 °C overmolding [6]

Cap reinforcement with epoxy [6] Cap reinforcement with metal [6]

Resonator + TFP + Cu 23µm

Sacrificial release hole

SiO2 cap

Reinforcement layer

Cap reinforcement with localized metal [7]

Molding epoxy

Cu 23µm

200µm

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TFP and low pressure specifications

Impact of package miniaturization Pressure increasing can come from:

Outgassing from materials inside TFP cavity (major factor)

µleak

Permeation

Possible schemes to reach low pressure TFP cavities: Optimized materials and outgassing process before sealing the cap

Implement getter materials

120

140

160

080

0

10

00

015

3045

1.0E-02

1.0E-01

1.0E+00

1.0E+01

1.0E+02

Surf

ace

/ V

olu

me

(µm

-1)

Length (µm)Height

(µm)

);( HeightLengthfVolume

Surface

MEMS wafer

Sensitive part

Top surface of the substrate - Passivation (SiO2 or SiN)

- Metal lines (Au, Al…)

Sensitive part of the device

Cap material (SiO2 or SiN) Sealing material

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TFP and low pressure specifications

Optimized materials and outgassing process before sealing the cap Materials outgassing properties, one of the key parameter [8]

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

Sample

1.E-03

1.E-02

1.E-01

1.E+00

1.E+01

1.E+02

0 50 100 150 200 250 300 350 400 450 500

Thermal treatments (°C/30')

Ou

tgas

sin

g (m

bar

.cm

) Al/TiN SiO2 TEOS

SiN SiO2 HDP

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TFP and low pressure specifications

Optimized materials and outgassing process before sealing the cap Chemical composition outgassing is another key parameter RGA analysis

mandatory

Sealed MEMS put in a High vacuum chamber

Open cavity in the chamber Analyze gas present

in the chamber

Mass spectrometer

2 benches available at Leti (Resolution N2 0.3 -Ar 0.02pmoles)

Best outgassing process can now be defined (max temperature, temperature ramp up, process time…)

Résiduel_[statique_série5-dynamique_série1]

0.0E+00

1.0E-11

2.0E-11

3.0E-11

4.0E-11

5.0E-11

6.0E-11

5 15 25 35 45

amu

I (A

)

Casse ampoule

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TFP and low pressure specifications

Implement getter materials

But getter effect depends on gas present inside the cavity (better the outgassing,

better the getter)

Tunable activation temperature (to fit with sealing process)

MEMS wafer

Sensitive part

Getter material is able to pump residual gases [13]

Getter properties

200

250

300

350

400

450

500

0,15 0,2 0,25

N2 sorbing capacity mbar.cm3/cm²

Acti

vati

on

tem

pera

ture

°C

AuSi

AuSn

Anodic

SDB

Leti catalog of getters

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TFP and low pressure specifications

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

Cap

Polymer

Hole

Release hole sealed

3.0 µm sealing layer

Al sealing materials [8]

Cap

SiO2 sealing

Sealing layers

Ti/Cu sealing materials [7]

Metallic sealing materials [9], [10]

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Pressure measurement: Q factor monitoring Few results published !

TFP pressure performances summary

TFP without getter TFP

Molecular Glass frit

AuSi

Anodic

AuSn

Wafer bonding techniques

[9]

TFP with getter

10-3 10-2 10-1 1 101 Pressure (mbar)

10-4

Thermal budget (oC)

102 103

[10]

[8]

[11]

[12]

950 oC

[6] [2] [5]

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Comparative cost analysis

Comparing between Wafer Level Packaging methods 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

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

Thin film packaging

1000µm X 700µm

Si cap packaging

1100µm X 800µm

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Comparative cost analysis : results

Thick Cu

ECD

Align /

Bonding

TSV DRIE

Align /

Bonding

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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

[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