Surface Forces in Nanomechanical Systems: Living on the Edge

Surface Forces in Nanomechanical Systems: Living on the Edge

J ProvineStanford University

2012-01-11Fermilab Colloquium

Outline

• Scaling in the micro/nanometer range• Introduction to several surface effects• Nanoelectromechanical Switches

– Application– As a nanoprobe– Device design for probing surface forces

• Conclusion

Outline



• Conclusion

A few quick words on scaling• We live in the m-cm world (100 to

10-2m)• MicroElectroMechanical Systems

(MEMS) and CMOS electronics circa 1990 1µm (10-6m)

• Current CMOS, thin film optical coatings, NEMS 10nm (10-8m)

• Carbon Nanotubes, atomic layer deposition coatings, self assembled monolayers 1nm (10-9m)

• Lattice constant of Si 5.4A (10-10m)• Fermilab…

The Dominance of Surface Effects

Volume 4/3 r3

Surface Area 6 r2

Surface Area:Volume 1/r

As the size of an object shrinks, the surface affects become more dominant because the object is becoming “all surface.”

Outline



• Conclusion

Some surface effects in nanodevices

• Photonics effects• Adhesion (geckos)• Nourredine smith wear/friction• Casimir Force

1. Make any material a good optical material

2. Get at the unique optical properties of specific materials

• Various unique optical material properties can be explored and exploited now because of great materials understanding.– Polariton Modes– Kerr Effect– Birefringence– Photoelectric transduction

Surface Effects in Photonics

• New ways to get excellent optical performance from a wide range of materials.

• Photonic Crystal and Subwavelength Grating design for allows a very wide range of materials to provide desired performance.

PCs come in many flavors

• Excellent test bed for some deep physics experiments (QED, surface physics, etc.)• Telecom and Photonic circuits.• Slow light.

Lin, et al, 2003

Kuchinsky, et al, 2002

Broadband Reflector Applications

M.C.Y. Huang, Y. Zhou, and C. Chang-Hasnain, Feb. 2007

I. Jung, S. Kim, O. Solgaard, Trans. 2007

• High temperature, high power handling.• CMOS compatible and integrable processing.

11

Monolithic Si Photonic Crystal Slab

Dielectric stack (DBR)

Slab photonic crystal

Monolithic photonic crystal

Materials for PC

20nm (2%) Increase

Air gap thickness change

Refractive index change

0.2 (5%) Increase

Hole radius

10nm (3%) Increase

Polysilicon thickness change

20nm (5%) Increase • Extensive testing has been

done for particular materials (Si, poly-Si, SiN, SiO2)

• But the key is ANY DIELECTRIC can be used to design PCs.

• Strong wavelength dependent guided or reflected modes can be created in materials to suit specific applications.

PC Fiber Tip Sensor Applications

• Biological, chemical, and mechanical sensors (such as accelerometers) at the end of an optical fiber can be useful for control and security applications

• The small size (125 µm diameter) enables them to penetrate tissue or veins for medical applications

• PCs at the tip of fibers can be used both for free-space and inline applications as a reflector, polarizer and filter

Fiber Tip Assembly

Pt weld of PC Direct weld of PC

Utilize direct weld of PC with ion beam as opposed to Pt weld to study impact of weld technique.

Index Sensing Experiment

3dB couplerPower meter

Optical spectrum analyzer

Fibe

r

PCWater/Solvent

Broadband source

Index Sensing Experi-ment

Experimental data

Refractive index calcu-lated from volume con-

centration

Refractive Index Sensing

Responsivity = DR.I./Dl = 0.04768 [nm-1] Sensitivity ≈ 4.8 x10-5 [pm-1]

Using an optical system (tunable laser, OSA) with picometer resolution Comparable to FBG refractive index sensors [W. Liang, A. Yariv et al, APL

2005]

Isopropanol concentra-tion increase in incre-ments of 30ml in DI

Water of 150ml

IEEE Nanophotonics 2009.

Temperature Sensing Experiment

Temperature Sensing Exper-iment

Experimental data

Temperature measurement taken while cooling from 80°C to room temperature

Temperature Sensing

Responsivity = Dtemp/Dl = 16.0858 [°C/nm] Sensitivity ≈ 0.016 [°C/pm]

Using an optical system (tunable laser, OSA) with picometer reso-lution

Almost an order better sensitivity than a FBG temperature sensor [A. D. Kersey et al., Fiber Grating Sensors Invited Paper, JLT 1997]

LEOS annual meeting 2009

19

[www.cnconveyorbelt.-com]

[www.tommcma-hon.net]

Harsh environments High voltage, high power ma-

chinery High temperature

[blog.mlive.-com]

[Onur Kilic]

Motion/Vibration/Explosion de-tection

Acoustic sensing Gyro/Acceleration

[www.af.mil]

[www.blueparrotevents.-coml]

Bio/chemical detection Biological/chemical agents Fluid, Gas sensing

[www.gallagher.com]

Structural Health monitoring Combustion chambers, Turbines Aircraft, wind turbines, bridges, dams, oil wells, pipelines Smart structures: Integrated fiber-optic sensors (aging, vi-

brations)

[newswhitehouse.-com]

[www.reuk.co.uk]

Impact & Applications

1m

Si beamSiC coating

Accessing a particular optical property in a novel material: SiC

Spitzer, et al, Phys Rev., 1959.

The optical properties of SiC have also been studied for a long time. Recently the interest has expanded because of the extremely strong mid-IR Phonon Polariton resonance.

Device FabricationSiO2

SiC

Bulk Si

Bulk Si

LPCVD SiC @ 800CLPCVD SiO2 for hard mask

Transfer photolithographic mask through SiO2 and SiC by RIERIE of SiC is HBr/HCl

Release membrane by XeF2 etch

Remove hard mask with HF dip

80 sidewall

Polariton Gap

Hole ArrayPatterned Film

Unpatterned Film

Theroretical simulation with FD3D Finite Difference Time Domain code.

a

d

t

Extraordinary Transmission

t = 4ma = 10.4md = 5.6m

Extraordinary TransmissionPolariton Gap


Unpatterned Film

t = 4ma = 10.4md = 5.6m

Polariton Gap

t = 1.5mPolycrystalline SiC

Experimental Data from FTIR

Unpatterned Film

d=5.6m

Unpatterned Film

Polariton Gap


Unpatterned Film

t = 4ma = 10.4md = 5.6m

Polariton Gap

t = 1.5m a = 10mPolycrystalline SiC



d=5.6m

d=3.9m

Polariton Gap


Unpatterned Film

t = 4ma = 10.4md = 5.6m

Polariton Gap

t = 1.5m a = 10mPolycrystalline SiC



Unpatterned Film

Provine, et al, OMEMS 2007

Reflection Spectra

t = 1.5m a = 10m

Polycrystalline SiCExperimental Data from

FTIR

d=3.9m

d=5.6md=4.8m

d=3.1m

Ongoing Experiments: A True Meta-Material

· Selective metal surface coatings. (Catrysse and Fan, Physical Review B, 2007)

Adhesion at the Nanoscale

Work between Autumn Lab (Lewis & Clark) &Kenny Lab (Stanford)

Casimir Force in Metals

Valid at 0 K and vacuum.

• Uncharged metals (equipotential) will still attract.

• Purely a quantum & geometrical effect.

• Hotly debated and studied because of the relation to the cosmical constant.

• At the nanoscale starts to have appreciable forces.

Casimir effect in Pt nanobeams

Nanobeam constructed from a single sheet of evaporated Pt (equipotential).Slices are made with ion beam and then released from unlying Si with XeF2.

Crystal orientation makes this a challenging study.

Outline



• Conclusion

Application: a downside of scaling

E. J. Nowak, IBM J. Res & Dev. 2002

• As modern CMOS electronics scales to smaller and smaller devices, the power consumption rising rapidly.

• Because of the ubiquitous computing ongoing (and being proposed) the amount of energy going to servers and even personal computing is becoming appreciable.

A Solution: Back to the Future

Babbage Analytical Engine c 1877

• Mechanical computing can be an answer to this issue because it can deliver zero off-state power consumption.

• Additional benefits:• Radiation hard

operation• Lower thermal

dependence• Is this a CMOS killer? NO• But it can have many

applications and certainly help with energy consumption. (see for instance Chen et al FPGA 2010.)

Implementing a NEM Swith

Examples of NEM Switches:Metallic Structures

Vertically actuated WColorado, Boulder

Laterally actuated RuSandia National Labs

Vertically actuated NiKAIST

Examples of NEM SwitchesConducting Ceramics

Vertically actuated TiNKAIST

Laterally actuated TiNStanford

Examples of NEM Switches:Semiconducting Structures

Vertically actuated poly-SiKAIST

Vertically actuated W coated SiGeCalifornia, Berkeley

Arbitrary NEM Logic Design Methodologies

• Only 6T relays required for all 3 generations• Our lateral 6T elemental logic block• New elemental block allows new design methodologies

G

Gate 1

Drain

Source 1

Source 2Gate 2

Beam

=Mold Layer(eg, Polysilicon)

=Insulating Layer(eg, Hafnium Oxide)

=Conductive Layer(eg, TiN or Pt)

Isolation

The Logic Element: 6T Relay

Y-Device Process Flow AA’ BB’ (a) Deposit 1um polysilicon on 1.5um oxide.

(b) Pattern polysilicon (mask 1).

Oxide

Substrate

Y-Device Process Flow AA’ BB’ (c) Deposit 20nm HfO2 via ALD.

(d) Blanket etch of HfO2.

Y-Device Process Flow AA’ BB’ (e) Deposit 20nm Pt or TiN via ALD.

(f) Etch Pt or TiN and pattern pads (mask 2).

Y-Device Process Flow AA’ BB’ (g) Pt or TiN wet etch for sidewall isolation (mask 3).

(h) Release in 49% HF followed by CPD.

Fabricated Device

Y-Device Switching Properties

0 5 10 15 2010-12

10-10

10-8

10-6

VGATE1[V]

Cur

rent

[A]

DrainSourceBeam

[S. Lee et al., Transducers 2011]

Current Flow (Source to Drain)

No Beam Current

Mechanical Delay Measurement

-0.06

-0.04

-0.02

0

0.02

0.04

0.06

-20-15-10

-505

101520

0.0E+00 1.0E-06 2.0E-06 3.0E-06

Drai

n Vo

ltage

[V]

Gate

Vol

tage

[V]

Time [s]

1.2µs

Easy, right? Not Always

ALD Platinum Coated Relay

• Large pull-out variation!• Adhesion force variations: asperity deformation

0 2 4 6 810p

100p

1n

10n

100nDr

ain C

urre

nt (A

)

Vgate (V)

Single device,multiple cycles

Other issues

• Desired improvement in– Total Lifetime– Uniformity between devices (same chip)– Uniformity between devices (different wafers)

• Concerns– Fabrication tolerance– Adhesion forces– Contact Resistance

Controlling the contact mechanism and apparent contact area:Existing designs

Point-surface contact mechanism with limited asperity-asperity contact

Flexible Contact Surface

Flexible surface-surface contact

Before pull-in

After pull-in

Point contact

Overdrive voltage

surface-surface contact

NEM Relays with improved contact properties

NEM Relays with improved contact properties

Mechanically robust designs with large overdrive voltage:

NEM Relays with Small FootprintUsing the coating as the main structural material:

200nm process and 50nm coating:Electrode length: 5µm Beam length : 5µmSource-gate gap: 100nmTiN coating: 50nm

0 1 2 30.06

0.07

0.08

0.09

0.1

0.11Gap-Voltage Response

Voltage (V)

Gap

(m

)

NEM Relays: 6T Relays 6T relays are sensitive to fabrication tolerances:

500nm process and 20nm coating:Beam length: 21um Gate length: 19umCoating thickness: 20nmBeam-gate gap: 560nmSource-drain gap: 460nmSource-drain tol.: 10nm (2%)

FEM simulations:First contact : 23VSecond contact: > 37V

V = 37VResult: Extensive overdrive often necessary.

NEM Relays: 6T Relays Insensitive to Fabrication Tolerance

Relays with flexible source-drain:

NEM Relays: New Designs (6T) Relays with flexible source-drain:

FEM Simulations:Displacements (V = 5V)


FEM Simulations:Contact pressure (V = 5V)

Result: Overdrive minimized for relay.


0 2 4 6 8 10 12 14 16 18 203.5

4.0

4.5

5.0

5.5

6.0

Voltage (V)

Cap

acita

nce

(pF)

0 2 4 6 8 10 12 14 16 18 200.0

0.1

0.2

0.3

0.4

0.5

Voltage (V)

Tip

Disp

lace

men

t (pF

)

FEM Simulations:

Switch vs. Nanoprobe• While the switching application is important and interesting, surface affects mean that simultaneously:o We need to understand surface forces more

accurately to optimize our switcheso The switches can operate as excellent

nanoprobes to determine what is happening.o Different materialso Different ambient conditionso Different designs to isolate particular material properties

NEM Relays: Reliability Material and surface characterization:

Young’s modulusStructural and air damping

Forc

e

Displacement

NEM Relays: Reliability Material and surface characterization:

Fracture stress

Adhesion force/Young’s modulus/Initial stress

NEM Relays: Reliability Material and surface characterization: Adhesion

Stiction to substrate Min

imum

ga

p

Maximum length

Stiction to side walls

Outline



• Conclusion

Take Home Messages• In general, nanofabrication has “grown up” to

the point we can make almost anything.– Lots of materials– A wide ranges of sizes (mm to A)

• While this opens up a wide range of new applications, it just as quickly allows (necessitates?) new science to be explored.

Acknowledgements• NEMS Logic Team (in particular Kamran Shavazipur)• Stanford

– Roger Howe Group– Philip Wong Group– Olav Solgaard Group

• UC Berkeley– Roya Maboudian Group– Tsu-Jae King Liu Group

• Center for Interfacial Engineering of MEMS (CIEMS)• DARPA and NSF for funding

Thank you.

Questions?

Documents

Surface Forces in Nanomechanical Systems: Living on the Edge