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-Ashwin VijaysaiNanocoatings

"Self-Cleaning Materials: Lotus Leaf-Inspired Nanotechnology“, August 2008, Scientific American.

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Is this familiar???

http://www.eye-doctor.ca/Content/eyeglasses/lenses/nikon/seecoat/seecoat.aspxhttp://www.pbma-fl.com/rain-repellent.html

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Overview• Significance of nano-coatings

• Why it is needed in MEMS– Anti- stiction (lube)– Improve lifetime

• Types of nano-coatings• Various chemistries

– Compare to choose the ideal– Observe aging of nano-coating

• Surface modification at TTU MEMS lab• Chloro-silane based SAM

– Hydrophobic coating- for MEMS

• {Alumina nanoparticle + Chloro-silane based SAM}– Superhydrophobic coating- for medical fabric

• Nano-coating characterization devices

Surface Engineering

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Significance of nanocoating

Increase life-time, re

liability and yield

Bett

er N

ano-

coati

ngs

performance

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Stiction based failure mechanisms• In MEMS is large• Stiction failure surface forces > spring forces• Common failure mechanisms due to stiction

are:– Capillary forces– Van der waals forces– Electrostatic forces

൬𝑺𝒖𝒓𝒇𝒂𝒄𝒆 𝑨𝒓𝒆𝒂𝑽𝒐𝒍𝒖𝒎𝒆 ൰

𝐸𝑖 = න 𝑐𝑖𝑧𝑛𝑖 ℎሺ𝑧ሻ𝑑𝑧∞0

* M. V. Spengen, “On the physics of stiction and its impact on the reliability of microstructures”, J. Adhesion Sci. Technol., Vol. 17, No. 4, pp. 563–582, 2003

Adhesion energy

ci = constant of force i depending on properties of surface and environmentn= power of interactionz = distance of separationh(z)=Distribution of distance between rough surfaces

• Design modification – dimple, curve, FEM• Roughening surface – selective etch, skewness• Anti-Stiction Coatings –

– Plasma deposited coatings– Self-Assembled monolayers (SAM coatings)– Vapor phase deposited coatings– Getters

Solutions for alleviating stiction

S.No. Coating type Adhesion energy

1 OTS 30 µJ/m2

2 FDTS 8 µJ/m2

3 Untreated ~56 mJ/m2

Poly-Silicon MEMS

U. Srinivasan, M. R. Houston, R. T. Howe and R. Maboudian, J. MEMS, 7, 252-260, 1998.

~1000 times decrease of

energy!

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Significance of nanocoating• In SUMMiT-V after final release step, liquid in-

between two poly-silicon layers can cause failure.

» Solution: Critical point drying________» Future Problem: Capillary condensation

• Anti-stiction coating (hydrophobic) minimizes capillary condensation.

• Hydrophobic – fear of water» eg: teflon cookware

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Significance of nanocoatingHigh performance devices

TRA

HPCD

Under actuation gap reduces ~.05µm

Flexures

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Vapor Phase deposition of SAMFor MEMS devices under fabrication the last few steps impact the yield, reliability & functionality:• Release step

– Critical point drying– Avoid release stiction

• Surface preparation– Surface cleaning (organic contaminants)– Asher/ Ashing/ UV-O cleaner

• Exposure– Exposed to precursor (nano-coating chemistry)

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• Significance of nano-coatings• Why it is needed in MEMS

– Anti- stiction (lube)– Improve lifetime

In addition to: • Dimples, roughness, straps, guides, etc

associated with design• Process involved in SUMMiT-V fabrication is

not in our hands• Surface chemistry involved in the conformal

treatment to all poly-silicon layers helps» Lower adhesion energy» Increase reliability lifetime of device

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Common types of nanocoatings1. Chlorosilanes2. Amines3. Alcohols4. Carboxylic acids5. Siloxanes6. Dimethylaminosilanes

* R. Ashurst et. al, “Vapor Phase Anti-stiction coating for MEMS”, Trans. on device and materials reliability, vol.3, iss. 4, pp. 173-178, 2003.

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Hydrolysis reaction1, some of the –OH groups participate in condensation reaction with –OH group on the oxidized surface of the silicon2, Cross polymerization occurs between the individual micelles

*Vapor-Phase Self-Assembled Monolayers for Anti-Stiction Applications in MEMS, Zhuang et al., J MEMS, 16, 6, 2007, pp. 1451-1460

*

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Amines and Alcohols• -Cl terminated Silicon surface is obtatined by

exposing –H terminated Si sample to Cl2 in vacuum at 80℃ temerature of sample

• Amine is –NH2 eq (1)• Alcohol is –OH eq (2)

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SiloxanesAnalog devices – ADXL accelerometers

1. During packaging step2. Small aliquot dispensed inside package3. Package is sealed4. Temperature of the chip is raised5. Liquid evaporates and forms organic

monolayerExample: Demethyl siloxane

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Carboxylic acidsTexas Instruments – DMD chip

1. Perfluorinated –n- alkanoic acids (PFxA) ‘’ decnoic acid (PFDA)

• Minimize friction coefficient• Minimize thermal decomposition

Solid PFDA inside hermetic sealSublimation of PFDA due to its vapor pressureHealing one of its kind in MEMS package

350 billion cycles

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Dimethylaminosilanes• PF8TAS• PF10TAS

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Synthesized from:FOTSFDTS

•Similar procedure as in chlorosilanes•But no need for water vapor/ water in the reaction

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• Types of nano-coatings• Various chemistries

– Compare to choose the ideal– Observe aging of nano-coating

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At TTU – RPX 550, IST system

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Reaction Chemistry• TMA (Alumina) Generation2Al(CH3)3 + 3H20 Al203 + 6CH4

• Linkerrix generation (Linking chemistry, that binds the TMA nano-particles)

SiCl4 + 2H20 Si02+ 4HCl

C2H4Cl6Si2 + 6H20 SiOx-(CH2)2-SiOx + 6HCl +3H20

C6H12Cl6Si2 + 6H20 SiOx-(CH2)2-SiOx + 6HCl +3H20

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Nomenclature

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Comparison of contact angle

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Comparison of Size

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Magnified images of superhydrophobic coating

SEM image of highly conformal coating

TEM image of alumina nano-particles.

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RPX 550 System and initial results* www.insurftech.com

LDDMS VDDMS

FOTS

* R. Ashurst et. al, “Vapor Phase Anti-stiction coating for MEMS”, Trans. on device and materials reliability, vol.3, iss. 4, pp. 173-178, 2003.

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• Surface modification at TTU MEMS lab• Chloro-silane based SAM

– Hydrophobic coating- for MEMS

• {Alumina nanoparticle + Chloro-silane based SAM}– Superhydrophobic coating- for medical fabric

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MEMS Tribogauge

Sense comb

Driving comb

Lateralaxis

Normalaxis

AD7747Eval

BoardLabView

MeasureDisplacement of

‘Head’, i.e,fingers (as ‘C’)Se

nse

com

b

𝑭𝒔𝒕𝒊𝒄𝒕𝒊𝒐𝒏 > 𝑭𝒓𝒚 𝑭𝒔𝒕𝒊𝒄𝒕𝒊𝒐𝒏 = 𝑭𝒓𝒚− 𝑭𝒚↓ ∆𝑭𝒂𝒅𝒅. = 𝑭𝒚↑↑ 𝒄𝒐 − 𝑭𝒓𝒚 𝑭𝒚↑↑ 𝒄𝒐 = 𝑭𝒚↑

TRIBOGAUGE MODEL

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‘δ’ D

isp

lace

men

t (µ

m)

D.C. Voltage (volts)0

3ApproachRetract

Maximum δ, C

Release Point

Maximum Displacement

Baseline Stiction Model‘C

’ Cap

acit

ance

(fF

)

𝑭𝒚↑ = 𝑭𝒓𝒚

𝑭𝒚↑ = ൬𝒏 𝜺 𝒕𝒈 ൰.𝑽𝟐

𝑭𝒓𝒚 = 𝒌𝒚.𝜹𝒚

,at contact of slider and scrubber𝑭𝒓𝒚 > 𝑭𝒔𝒕𝒊𝒄𝒕𝒊𝒐𝒏

𝑭𝒚↓ = 𝑹𝒆𝒕𝒓𝒂𝒄𝒕 𝒆𝒍𝒆𝒄𝒕𝒓𝒐𝒔𝒕𝒂𝒕𝒊𝒄 𝒇𝒐𝒓𝒄𝒆 𝒇𝒐𝒓 𝒏𝒐𝒓𝒎𝒂𝒍 𝒅𝒓𝒊𝒗𝒊𝒏𝒈 𝒄𝒐𝒎𝒃𝒔

∆𝑭𝒂𝒅𝒅. = 𝑵𝒐𝒓𝒎𝒂𝒍 𝒍𝒐𝒂𝒅 𝒃𝒆𝒚𝒐𝒏𝒅 𝒕𝒉𝒆 𝒄𝒐𝒏𝒕𝒂𝒄𝒕 𝒐𝒇 𝒔𝒍𝒊𝒅𝒆𝒓 𝒂𝒏𝒅 𝒔𝒄𝒓𝒖𝒃𝒃𝒆𝒓

𝑭𝒚↑↑𝒄𝒐 = 𝑴𝒊𝒏𝒊𝒎𝒖𝒎 𝒇𝒐𝒓𝒄𝒆 𝒘𝒉𝒆𝒏 𝒔𝒍𝒊𝒅𝒆𝒓 𝒊𝒔 𝒊𝒏 𝒄𝒐𝒏𝒕𝒂𝒄𝒕 𝒘𝒊𝒕𝒉 𝒔𝒄𝒓𝒖𝒃𝒃𝒆𝒓

𝑭𝒚↑ = 𝑨𝒑𝒑𝒓𝒐𝒂𝒄𝒉 𝒆𝒍𝒆𝒄𝒕𝒓𝒐𝒔𝒕𝒂𝒕𝒊𝒄 𝒇𝒐𝒓𝒄𝒆 𝒇𝒐𝒓 𝒏𝒐𝒓𝒎𝒂𝒍 𝒅𝒓𝒊𝒗𝒊𝒏𝒈 𝒄𝒐𝒎𝒃𝒔

𝑭𝒓𝒚 = 𝑹𝒆𝒔𝒕𝒐𝒓𝒊𝒏𝒈 𝒔𝒑𝒓𝒊𝒏𝒈 𝒇𝒐𝒓𝒄𝒆 𝒇𝒐𝒓 𝒏𝒐𝒓𝒎𝒂𝒍 𝒔𝒆𝒏𝒔𝒆 𝒂𝒏𝒅 𝒅𝒓𝒊𝒗𝒊𝒏𝒈 𝒄𝒐𝒎𝒃𝒔

n : number of comb fingers = 160

ε: 8.854*10-12 F/m

t: 7*10-6 m

g: 0.5*10-6 m

V: approach voltage volts

Ky: Spring constant of sense and driving segments of normal axis

δy: Displacement of the slider, in µm

𝑭𝒚↓ = 𝑭𝒓𝒚

𝑭𝒚↓ = ൬𝒏 𝜺 𝒕𝒈 ൰.𝑽𝟐

𝑭𝒓𝒚 = 𝒌𝒚.𝜹𝒚

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Adhesion Static Friction, Static friction coefficient

In-use stiction

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Nano-coating characterization devices/ techniques

Atomic Force Microscope:1. Topography2. Monolayer topography3. Force – Distance measurement4. Lateral force measurement

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Backup

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Static friction is friction between two solid objects that are not moving relative to each other. For example, static friction can prevent an object from sliding down a sloped surface. The coefficient of static friction, typically denoted as μs, is usually

higher than the coefficient of kinetic friction.The static friction force must be overcome by an applied force before an object can move. The maximum possible friction force between two surfaces before sliding

begins is the product of the coefficient of static friction and the normal force: .

Kinetic (or dynamic) friction occurs when two objects are moving relative to each other and rub together (like a sled on the ground). The coefficient of kinetic friction is typically denoted as μk, and is usually less than the coefficient of static friction for the same materials

wikipedia

Types of frictionWith respect to motion

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A1) Super-hydrophobicity is a combination of roughness, coverage (aerial density) and surface energy. So alumina nano-particles are very important for roughness and how you distribute the nano-particles over the surface. But you need to combine this with low surface energy of the Phobix. A2) The boulder formation that IST reports was observed when using amine – epoxy adhesion versus a linkerrix (oxide) adhesion. In the ATETATETA is (A=amine step / T=TMA aluminum oxide / E=Epoxy) a sequence. The durability was improved with Amine-T-Epoxy reaction schemes. However, IST’s highest durability was observed with a ALD layer.