Developments in MEMS Packaging Presented to IEEE-CPMT Alissa M. Fitzgerald, Ph.D. | 13 April 2011
Overview • • • •
About us What are MEMS? MEMS-specific packaging issues MEMS process technologies important to IC
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About AMFitzgerald
Company Mission
We turn your ideas into silicon.
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Fully integrated services: concept to production
Technology Strategy
• • • •
Design Simulation
Prototyping
Testing
Foundry Transfer
Complete project management Feasibility and cost analysis Design optimization using simulation Development on 150 mm wafers, small batches – Prototype fabrication with own staff engineers at UC Berkeley’s Microlab
• Test system development • Packaging, system integration • Foundry selection, tech transfer and ramp-up Page 5 Presented to IEEE-CPMT/Silicon Valley
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MEMS design and process expertise
Technologies we have developed: • • • • • • • • • • •
Piezoresistive devices Piezoelectric (AlN and ZnO) devices Electrostatic structures Solar cells Passive microfluidics Electrophoretic pumps Mold masters Gratings, phase shift lenses etc. PDMS, SU-8 structures Mechanical dummies for package reliability testing Custom test systems
Over 70 clients served
Application areas: • • • • • • • • • • • • • •
Chemical sensing Materials characterization Medical implant Medical diagnostics Pressure sensing Filtration products Laser/ Infrared/ Visible optics Chip cooling Cell culture Radiation sensing Microphones Gas flow metering Multi-chip modules Solar
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Standard TSV available for prototypes • Silex Microsystems Sil-Via® – Already in volume production for consumer electronics – “Via-first” process – Solid via: mechanically stable and robust Wafer surface contact hole opening
Wafer surface passiv ation
Dielectric filled trenches isolating via connections from bulk silicon Low resistiv ity bulk wafer silicon
Low resistiv ity silicon electrical connection through wafer
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Modeling and design optimization • ANSYS Multiphysics R13 • Matlab • Proprietary fracture prediction • Intelligent use of simulation to minimize risk and reduce fab cycles
Package-induced stresses
– Management of uncertainty in MEMS material properties
• Design exploration and performance optimization
Magnetic field of inductor coils Page 8 Presented to IEEE-CPMT/Silicon Valley
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Technology strategy • • • • • •
Device feasibility Manufacturing cost models Technology readiness Patent landscapes Development roadmaps Due diligence
Customized workshops on MEMS
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What are MEMS and how are they different from IC?
What are MEMS? Airbag sensors (1980)
• Micro Electro Mechanical Systems – Not a platform device technology – But a powerful manufacturing technology for miniaturization
• Semiconductor process heritage
Source: Ed Phillips
Airbag sensors (2005)
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Challenge of MEMS development • High technical complexity – Coupled physics – Moving parts – Environmental exposure – Test and packaging challenges
Microvision Pico-P
TI DLP pixels
15 um
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Why MEMS are exciting for so many applications • Smaller, better, cheaper
Integrated Pressure Sensor
– But not always all three
• Ease of electronics integration enables sophisticated capabilities in small form factor: – Multiple sensors – Signal processing and analysis Stacked MEMS – Telemetry capability and ASIC chips, wirebonded – Low power • Multi-chip module preferred to monolithic integration
MEMS sensor
Source: IMD
Source: Chipworks/Kionix Page 13 Presented to IEEE-CPMT/Silicon Valley
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MEMS are not ICs! • Parallels to IC can be misleading and dangerously naive – IC design to product < 18 months – Enabled by well established processes, design rules, sophisticated simulation software – Competitive wafer costs – Packaging standards and automation • MEMS design to product timeline > 5 years typical – Lack of sophisticated simulation tools and process standards Solutions evolving slowly
– Wafer costs vary widely – Packaging challenges are huge and solutions are considered a proprietary advantage Can be > 50% of unit cost Page 14 Presented to IEEE-CPMT/Silicon Valley
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Today, most MEMS go into consumer electronics • Current MEMS devices in consumer electronics – Accelerometers: games/apps, user interface – Gyroscopes: games/apps, user interface – Microphones: size and cost reduction (assembly)
• Until 2007, inkjet, DLP, and automotive dominated MEMS markets
Thanks to the iPhone, I can finally explain what I do for a living to non-engineers!
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Other consumer electronics that use MEMS
Cameras
GPS navigation
(image stabilization, microphones)
Laptops (shock isolation)
Game consoles Portable projectors Page 16 Presented to IEEE-CPMT/Silicon Valley
(user interface)
Sports performance/safety © AMF 2010-2011
Near-term MEMS developments: Mobile phones
Accelerometers: image stabilization, shock protection, navigation
Gyroscopes:
Microphones:
navigation
multiple devices for noise cancellation
Pressure sensors
Multiple Cameras
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Near-term MEMS developments: Mobile phones
Speakers
Oscillators
Magnetometers & IMUs: navigation, games
Pico-projectors Low power displays
RF filters
Micro-lenses: focus enhancement
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MEMS-specific packaging issues
MEMS-specific packaging issues • Some MEMS need to be open to the environment – Pressure sensors, microphones – Chemical, fluidic sensors
• Others need to be hermetically sealed, sometimes with a buffer gas and/or anti-stiction agent – Accelerometer – Gyroscope – Oscillators
• Delicate structures • MEMS sensors can sense package strain! – Package-induced stresses cause zero offset, drift, non-linear behavior Page 20 Presented to IEEE-CPMT/Silicon Valley
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Pressure sensor packaging: open to environment
Measurement Specialties
Package form factor varies according to pressure range and application
VTI
GE Sensing
AST Page 21 Presented to IEEE-CPMT/Silicon Valley
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Inertial sensor packaging: MEMS+ASIC
Need to integrate ASIC AND hermetically seal MEMS
Chipworks: STMicro 3-axis accel
InvenSense gyroscope
Chipworks: Bosch 3-axis accel
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SiTime MEMS oscillator: hermetic sealing
Hermetic encapsulation using thick epi-poly “MEMS first” process, CMOS can be added later
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Delicate, released structures create handling problems
Microvision Pico P
Texas Instruments DLP
FLIR bolometer
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MEMS packaging considerations • Variety of shapes and form factors make standardization difficult – Package design and integration must be part of MEMS R&D effort – Some capped/sealed MEMS can be packaged like IC chips and leverage IC packaging methods
• Packages can provide competitive and cost advantages – Lots of IP generated in solving MEMS packaging problems
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MEMS process technologies now used in IC
The Bosch DRIE Process A cyclic process alternating between etch and passivation F + ions
Mask
SiF 4
SF6 Plasma
Si -CF 2-
C4F8 Plasma
Passivation
Si
SF6 Plasma
Scalloping
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Deep silicon etch (DRIE) • Patented by Bosch, licensed to tool manufacturers – Significant process technology in MEMS, and now IC
• Aspect ratios pushing 50:1 • Silicon etch rates 1- 50 um/min
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TSV and isolation trenches enabled by silicon DRIE
ST-Ericsson imec
Alchimer
Silex Microsystems
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Sacrificial etch: make MEMS from CMOS wafers • Anhydrous vapor HF
HF vapor etch
– Oxide etching
• XeF2 – Isotropic silicon etchant, does not attack metals
• Release MEMS structures made in a CMOS process
Chipworks: ADI accel
XeF2 etch
Akustica microphone
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Front-to-backside lithography • Registration of front and backside patterns essential in MEMS
Implanted piezoresistors and metal contacts
Membrane (5-20 um typ.)
– Pressure sensor
• Now useful in IC – Interposers – Wafer bonding – TSV
Cavity etched from backside
Basic pressure sensor cross-section
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Wafer bonding • Anodic: CTE-matched glass to silicon • Glass frit • Metal eutectics • Plasma-activated • Adhesives – low temperature, temporary • Front-to-back pattern registration within 2 um
EVGroup bonder
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MEMS process technologies migrating to IC • Through silicon via • 3D integration – chip stacking and bonding • Trench isolation
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Summary • Huge variety in MEMS architectures and applications create endless packaging challenges • TSV and 3DIC people: welcome to our mechanical world! – Stress, CTE mismatch problems - familiar MEMS problems
• MEMS-specific processes now enabling semiconductor technologies
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