aluminium nitride piezoelectric technology for next ... · scandium doped aln now being...

©2015 SPTS Technologies - Confidential & Proprietary

David Butler

VP of Product Management & Marketing

Aluminium Nitride Piezoelectric

Technology for Next Generation MEMS

2

This presentation and the information contained within it is the property of SPTS Technologies and is confidential. Any duplication, disclosure, distribution, dissemination or copying

of this presentation or its contents or use for any purpose other than that for which it is supplied is strictly prohibited, without the prior written consent of SPTS Technologies. © 2015 SPTS Technologies

■ PiezoMEMS in high volume production – BAW

■ Emerging applications for piezoMEMS

■ Microphones

■ Ultrasonic transducers: PMUTS

■ Other applications

■ Processing challenges for AlN MEMS

■ Thickness control

■ Crystal orientation

■ Stress control

■ Summary

Contents


Volume PiezoMEMS Production - BAW

4



■ Two types of filter in any smartphone

■ Surface Acoustic Wave – SAW

■ Bulk Acoustic Wave – BAW

■ BAW uses AlN piezoelectric layers

■ Material “rings” at a set frequency – a filter

■ BAW used for applications > 1.8GHz

■ First appeared with 3G

■ BAW devices manufactured in standard Si fab

■ Avago & Qorvo dominate the market, @90% share

■ Epcos growing fast from a small base

AlN PiezoMEMS Ship in Huge Volumes

5



■ BAW filters shrink as frequency rises

■ Big benefit for 4G/LTE (2.3-2.7GHz and 3.5GHz in future)

■ Low insertion losses, less power consumption

■ Steeper skirts prevent interference

■ Pack signals into smaller bands

Why BAW?

6



■ 41 separate RF bands supported by a smartphone

■ Different countries, different carriers, GSM to 4G

■ 6 to 8 BAW in a smartphone

■ First shipped in ~2005. 8 Billion AlN BAW will ship this year

3G, 4G, 5G…. BAW Growth

7



■ SMR - Solidly Mounted Resonator. Used by Qorvo, Epcos

■ Multi-pair Bragg reflector under AlN

■ W/SiOx bi-layer stack

■ Reflects acoustic wave back into resonator – more efficient

■ FBAR – Film Bulk Acoustic Resonator. The other 50%

■ Replaces Bragg reflector with air gap

■ Used by Avago

SMR BAW - @50% of the Market


Microphones

9



■ Up to 4 MEMS mics in a modern smartphone

■ Large arrays give stronger signal, better pick-up

■ To support large arrays need:

■ High signal to noise ratio (SNR)

■ Tight tolerance, mic to mic

Microphone Content Increasing

iPhone iPhone 3 iPhone 3GS iPhone 4 iPhone 5

1 ECM

microphone

1 ECM

microphone

1 ECM

microphone

2 high SNR

MEMS

microphones

+ 1 MEMS

microphone

for headset

3 high SNR

MEMS

microphones

+ 1 MEMS

microphone

for headset

http://www.google.co.uk/url?sa=i&rct=j&q=&esrc=s&frm=1&source=images&cd=&cad=rja&uact=8&ved=&url=http://www.resimbul.com/iphone/iphone1.xhtml&ei=McGbVcuTEaKs7Aa-_oe4Aw&bvm=bv.96952980,d.ZGU&psig=AFQjCNEpQebJGoFM0rzV2m1-OTkA8TMmSA&ust=1436357297494948

http://www.google.co.uk/url?sa=i&rct=j&q=&esrc=s&frm=1&source=images&cd=&cad=rja&uact=8&ved=&url=http://www.resimbul.com/iphone/iphone1.xhtml&ei=McGbVcuTEaKs7Aa-_oe4Aw&bvm=bv.96952980,d.ZGU&psig=AFQjCNEpQebJGoFM0rzV2m1-OTkA8TMmSA&ust=1436357297494948

http://www.google.co.uk/url?sa=i&rct=j&q=&esrc=s&frm=1&source=images&cd=&cad=rja&uact=8&ved=0CAcQjRw&url=http://www.cultofmac.com/137752/samsung-is-now-shamelessly-ripping-off-the-design-of-the-4-years-old-iphone-3g-photo/&ei=YMKbVZ3dOO607QbDppPAAg&bvm=bv.96952980,d.ZGU&psig=AFQjCNFPH4R38Wd361A0-wJNuMQUkoXrxA&ust=1436357549764529

https://www.google.co.uk/url?sa=i&rct=j&q=&esrc=s&frm=1&source=images&cd=&cad=rja&uact=8&ved=0CAcQjRw&url=https://support.apple.com/kb/SP655&ei=FMObVa3OFseU7QaltoO4Ag&bvm=bv.96952980,d.ZGU&psig=AFQjCNEr5I1ShrvBPG3BxiP2yWpkZQb7Eg&ust=1436357647632277

https://www.google.co.uk/url?sa=i&rct=j&q=&esrc=s&frm=1&source=images&cd=&cad=rja&uact=8&ved=0CAcQjRw&url=https://support.apple.com/kb/SP655&ei=FMObVa3OFseU7QaltoO4Ag&bvm=bv.96952980,d.ZGU&psig=AFQjCNEr5I1ShrvBPG3BxiP2yWpkZQb7Eg&ust=1436357647632277

10



■ An array focuses on the desired signal

■ Reduces the undesired signal

■ Can locate the target signal in a room

■ Voice control of home appliances

■ Noise cancellation in a phone

■ Best performance needs well matched mics

■ AlN matching is well understood from BAW fabs

Microphone Arrays

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■ Capacitive mic read signal across two near surfaces

■ Surfaces can stick together. Sensitive to dust & moisture

■ In a piezo mic, sound deflects the AlN plates

■ Creates strain, converts into electrical signal

■ Easier to control thickness of plates than a gap

AlN PiezoMEMS Microphones

Capacitive PiezoMEMS

(Vesper)

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■ Suspend part above silica dust

■ Average particle size 6um

■ Blow dust with compressed air for 10 secs

■ Gently blow out dust from several cm

PiezoMic is Dust Proof

Capacitive microphone Piezo microphone (Vesper)


Ultrasonic Transducers

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■ A PMUT looks for ultrasonic echoes

■ Echo strains the AlN membrane and generates a charge

■ Electronics read that charge and amplify it

■ Multiple uses

■ Fingerprint sensor

■ Patterns on a finger give different echoes

■ Chemical sensor

■ Absorbed chemicals change mass of AlN transducer

■ Change frequency of response

■ Fat sensor – personal health

■ Blood screening, bone density.. Personal sonogram

■ Gesture control

■ Ultrasonic signal travels through air

■ Echo from fingertips hovering above phone screen

Piezoelectric Micromachined Ultrasonic

Transducer - PMUT

15



■ Capacitive sensors rely on pushing surfaces together

■ A ridge closes the gap, valley does not. Can be fooled by dirt, moisture

■ A PMUT sensor uses ultrasonic reflections

■ Finger valleys contain air – strong echo

■ Ridges give weak echo

■ Generate a 3D image, with some depth information

■ Impervious to moisture, grime. More foolproof

■ Rapidly growing market – 17% CAGR to $14B by 2020

PMUT Fingerprint Sensors Advantage

UC Davis/Chirp

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■ 24x8 PMUT array

■ AlN piezo deposited on wafer, bonded to CMOS wafer

■ 220um well etched from back of MEMS to leave a Si/AlN membrane

■ Generate ultrasonic pulse

■ Apply voltage to deflect AlN membrane & emit pulse

■ Read echo from finger

■ Reflected wave deflects AlN – generates charge

UC Davis/Chirp Fingerprint Sensor

Finger

Valleys – big echo

Ridge – small echo

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■ Energy harvesters

■ 10’s of uWatts potential with AlN

■ More power needs bigger die – 300mm?

■ Speakers

■ Inertial sensors

■ DETF – Double ended tuning fork. Made of AlN

■ Motion imparts momentum to proof mass - @100ng

■ Strains DETF, either compressive or tensile

■ Strain converted to a signal – detect direction of motion

■ BUT huge installed base for capacitive, tough to displace

Other Potential Applications

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■ AlN is the CMOS fab-friendly choice

■ Able to integrate into monolithic device – CMOS MEMS

■ Mature, understood, cost-effective, working

■ Scandium doped AlN now being investigated

■ Higher coupling coefficient than AlN

■ Very expensive material

■ Lead Zirconium Titanate - PZT

■ @10X higher coupling coefficient than AlN

■ Not easy to deposit

■ Not allowed in CMOS fab

Other Piezoelectric Materials?

Film stress, MPa

AlN

Sc-AlN

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■ Very similar to that required by BAW makers

■ Excellent non-uniformity

■ Thickness dictates frequency of “ring”

■ Best uniformity required to the wafer edge

■ (In BAW, ion beam trimming tunes the layer)

■ The “right” texture, or crystal orientation

■ Measured by XRD, want Full Width Half Max (FWHM) of <1.5°

■ If >2.5°, piezoelectric effect largely lost

■ Stress – the most critical. Across wafer and through layer

■ Stressed membranes can break

■ Stress change through the film can cause a cantilever to curl

Processing Challenges for AlN MEMS


AlN Piezo Processing

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■ Best performance dictates single wafer processing

■ Isolated modules. AlN sensitive to vacuum contamination

■ Usually configured with pretreat, electrode & AlN dep

AlN Processing Basics

RHSE

Electrode

(Mo, W, AlCu) AlN

Sigma fxP: PVD cluster system for AlN & electrodes

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AlN PVD Chamber

Reactive sputtering Ar & N2 mix

Platen RF bias for stress control

@150 to 250C

Rotating for NU

Pulse DC on pure Al target AlN is an insulator

Rotating magnetron x, y and z adjustment for NU

Rotation speed for NU

Shutter “no-wafer” pasting with pure Al

Particle & process control

Shielding Roughened for low particles

Cryopump AlN sensitive to contamination

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■ Deposition rate naturally falls as target ages

■ Target to wafer spacing increases as target consumed

■ Changes in material ejection angles as surface erosion profile develops

■ Software automatically adjusts target power or process time

Adjusting for Target Life

y = -8E-11x4 + 2E-07x

3 - 0.0001x

2 + 0.0086x + 99.5

95.0

95.5

96.0

96.5

97.0

97.5

98.0

98.5

99.0

99.5

100.0

0 100 200 300 400 500

Acc Power (kWhr)

To

oli

ng

Fac

tor

(A/k

W/m

in)

24



■ Rotating magnetron generates eddy currents in target

■ Opposes the magnetic field

■ Biggest effect at edge of target, faster magnet sweep

■ Causes change in target erosion, centre to edge

■ Slow magnetron spin speed in recipe, to balance effect

2nd Order Uniformity Adjustment

0

0.5

1

1.5

2

2.5

3

25 30 35 40

Uniform

ity %

1 S

igm

a

Rotation Speed

Magnetron rotation speed, 200mm wafers

25



Tight Wafer to Wafer Thickness Control

AlN thickness repeatability over 350 200 mm wafers 0.12% 1σ

26



■ As volume ramps, makers moving to 200mm

■ Steep reduction in AlN thickness at wafer edge on 200 mm wafers

■ Difficult/costly for edge trim correction

■ Common problem for 200 mm AlN using planar rotating magnetron

WIW NU Improvement at 200mm

27



New Uniformity Hardware

Original BKM

B-MAX Process

For one customer, B-MAX gave

a 10-15% increase in yield

200mm wafer

3mm EE

0.24% 1 sigma

28



■ Goal is highly oriented AlN to get piezoelectric effect

■ Grows best on smooth surfaces

■ Use reactive etch before electrode

Crystal Orientation

Smoothing Etch AlN FWHM

“Rough” oxide underlayer No 3.0°

“Rough” oxide underlayer Yes 1.4°

With smoothing etch

Al FWHM = 2.4°

Roughness = 2.2 nm

AlN FWHM = 1.4°

Without smoothing etch

Al FWHM = 5.0°

Roughness = 30 nm

AlN FWHM = 3.0° X

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■ RF Bias used to control stress of AlN film

■ More ion bombardment -> more compressive film

■ Control Options

■ Forward Power

■ DC Bias

■ Film Thickness

■ Increasing thickness requires

higher bias for set stress level

AlN Stress Control

Ar+ e- Ar+ e- e-

Matching

Unit 13.56 MHz

Generator

-500

0

500

1000

0 20 40 60 80 100

Platen power (W)

Str

ess (

MP

a)

0

50

100

150

200

0 5 10 15 20 25 30 35 40 45 50

Wafer Count

Str

es

s (

MP

a)

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■ AlN piezoMEMS in production since 2005

■ Bulk acoustic wave filters in every smartphone

■ Growth will continue with demands for bandwidth

■ New applications emerging, low voltage MEMS

■ More robust microphones

■ Multiple uses for ultrasonic transducers

■ Wearables, IoT… PiezoMEMS activity increasing

■ Processing demands similar to BAW

■ Very tight thickness control with minimum EE

■ More emphasis on stress

■ Active CIP programs at equipment vendors

■ New piezo materials and integration

■ Hardware and process optimization

Summary

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■ Matt Crowley - Vesper

■ David Horsley – Chirp and UC Davis

■ David Schnaufer - Qorvo

■ Gianluca Piazza – Carnegie Mellon

■ Emmanuel Quevy – consultant. Founded Si Labs

■ Xavier Rottenburg – imec

Thanks to…

aluminium nitride piezoelectric technology for next ... · scandium doped aln now being...

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