ME 645: MEMS: ME 645: MEMS: Design Fabrication Design Fabrication - - PDF document

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ME 645: MEMS: ME 645: MEMS: Design Fabrication Design Fabrication

Lecture 14: Stiction in MEMS

Design, Fabrication Design, Fabrication and Characterization and Characterization

P.S. Gandhi P.S. Gandhi Mechanical Engineering Mechanical Engineering IIT Bombay IIT Bombay

PRASANNA S GANDHI gandhi@me.iitb.ac.in PRASANNA S GANDHI gandhi@me.iitb.ac.in

Acknowledgments: G.M.VINOD & R.GIRISH

Contents Contents

I t d ti I t d ti

• Introduction

Introduction

• Friction Vs

Friction Vs Stiction Stiction

• Classification

Classification

• Causes of

Causes of Stiction Stiction

• Capillary Force

Capillary Force

• Hydrogen Bridging

Hydrogen Bridging

• Electrostatic Attraction

Electrostatic Attraction

• Electrostatic Attraction

Electrostatic Attraction

• Vander Waals force

Vander Waals force

• Comparison of Forces Causing

Comparison of Forces Causing Stiction Stiction

• Stiction

Stiction Reduction Reduction Methods Methods

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I ntroduction I ntroduction

Why Surface Forces Dominate at Micro Level? Why Surface Forces Dominate at Micro Level?

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Fs: Surface Forces, Fs: Surface Forces, proportional to L proportional to L2

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Fb Fb: Body Forces, : Body Forces, proportional to L proportional to L3

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L: Characteristic Dimension of the part L: Characteristic Dimension of the part L’ : New value of L L’ : New value of L Let the part be scaled down ‘r’ times, Let the part be scaled down ‘r’ times, i.e i.e; L/L’ = r ; L/L’ = r Fs’ = Fs /r Fs’ = Fs /r2 Fb Fb’ = ’ = Fb Fb /r /r3 Fs’/ Fs’/Fb Fb’ = (Fs/ ’ = (Fs/Fb Fb) x r, r> 1 for scaling down. ) x r, r> 1 for scaling down. Therefore, Therefore,

Ratio of Ratio of Surface Forces Surface Forces to to Body forces Body forces increases linearly as increases linearly as size reduces size reduces 5 cm to 50 micron : : : ::::::> 1000 times increase in Fs/ 5 cm to 50 micron : : : ::::::> 1000 times increase in Fs/ Fb Fb

Stiction Stiction: St : Static

atic Fr Friction

iction

• The threshold force requirement

to overcome static cohesion/ adhesion between two surfaces

• Surface adhesion forces are

hi h th h i l t i higher than mechanical restoring forces at microscale

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Friction Friction Stiction Stiction

Comparison Comparison

• Occurs between two objects

Occurs between two objects in contact with each other in contact with each other with a normal force Fn with a normal force Fn between them. between them.

Friction Friction Stiction Stiction

Ff= μf Fn

• Occurs even

Occurs even without without a pre a pre-

• existing force on the members

existing force on the members

• Stiction

Stiction force remains even force remains even

• If the normal force of

If the normal force of friction is removed the force friction is removed the force

• f friction will be gone
• f friction will be gone
• Independent of area of

Independent of area of contact contact after the initial force that after the initial force that brought them in contact is brought them in contact is removed removed

• Proportional to the area of

Proportional to the area of contact contact

Release Stiction

Surfaces locked by the presence of

thin film of liquid remaining after drying process

In use Stiction Liquid Film

Occurs when Micro Structures come

into contact during operation (may not be because of liquid)

In Use Contact

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Causes of Causes of Stiction Stiction

• Contact occurs due to

Contact occurs due to – – – Capillary Forces Capillary Forces – Electrostatic forces Electrostatic forces – Shock loads Shock loads – Insufficient stiffness of supports Insufficient stiffness of supports – Drying process after HF release of Sacrificial layers Drying process after HF release of Sacrificial layers – Desired contact by design Desired contact by design

• Forces Contributing to

Forces Contributing to Stiction Stiction – Capillary forces Capillary forces – Hydrogen Bridging Hydrogen Bridging – Electrostatic forces Electrostatic forces – Van Van der der Waals Waals forces forces

F

Capillary Force Capillary Force

σ= Surface tension force F = Spring force A = Wetted area

Internal fluid pressure due to surface tension, P = (2σ Cos θ)/ t

θ σ F t

Spring force = Internal fluid pressure x Area of plate F = ((2 σ Cos θ) /t )x A 2 σ A Cos θ t F =

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Hydrogen Bridging Hydrogen Bridging

• Hydrophilic Silicon Surfaces absorb water to a small depth

Hydrophilic Silicon Surfaces absorb water to a small depth y p p y p p below their surface layer. below their surface layer.

• Close contact of two hydrated surfaces forms hydrogen bonds

Close contact of two hydrated surfaces forms hydrogen bonds between oxygen and hydrogen atoms of the adsorbed water between oxygen and hydrogen atoms of the adsorbed water layers. layers.

• Water molecules, at such small scales, act as a kind of glue.

Water molecules, at such small scales, act as a kind of glue.

Electrostatic Electrostatic attraction attraction

• Can act over a distance and bring two micro objects into

Can act over a distance and bring two micro objects into g j g j contact contact

• If there is a dielectric material between two bodies in contact,

If there is a dielectric material between two bodies in contact, the charge between them may remain for some time. the charge between them may remain for some time.

• The force of attraction in such a case is given by

The force of attraction in such a case is given by – –

F = Є A V2 2 h2

• After the contact occurs, the electrostatic charges will

After the contact occurs, the electrostatic charges will dissipate or equalize based on the material properties, dissipate or equalize based on the material properties, resulting in no net force. resulting in no net force.

2 h2

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Van Van der der Waals Waals forces forces

• Weak forces arising due to induced polarization of molecules

Weak forces arising due to induced polarization of molecules g p g p

• r grains of polycrystalline material.
• r grains of polycrystalline material.
• For two parallel surfaces separated by a distance ‘d’ , the

For two parallel surfaces separated by a distance ‘d’ , the Vander waals force ‘Fv’ is given by [2] Vander waals force ‘Fv’ is given by [2]

Fv = A 6π d3 d≤ 30nm Fv = B d4 d> 30nm A , B : Hamaker Constants for the material

Comparision Comparision of Forces

• f Forces

Causing Causing Stiction Stiction

Figure by MIT OCW. After Komvopoulous, K. "Surface engineering and microtribology for microelectromechanical systems." Wear 200 (Dec, 1996): 305- 327.

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Stiction Stiction Reduction Reduction Methods Methods

Choosing Low Surface tension liquid to rinse the Microstructures Using dry gas etchants Making Contact surfaces rough; thereby reducing actual contact

area

Providing Low Surface energy coating on microstructure surfaces to Eliminate or reduce capillary forces Eliminate Chemical bonding Reduce electrostatic forces Ref: http://www.memsurface.com/stiction

Examples

• 1. DMP uses flourinated fatty acid self assembled monolayer (SAM) on the Aluminium oxide surfaces
• 2. Analog Devices Inc. use thermal evaporation of Silicon Polymeric materials at the packing stage of

their inertia sensors

Stiction Stiction Reduction Reduction Methods Methods

Choosing a suitable drying process to avoid solid liquid interface

g y g p q

Sublimation Drying Supercritical Drying Liquid Solid Pressure Temperature Vapour

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References References

1 Dongmin Wu Nicholas Fang Cheng Sun Xiang Zhang Stiction 1. Dongmin Wu ,Nicholas Fang, Cheng Sun, Xiang Zhang, Stiction Problems in releasing of 3D microstructures and its solution, Sens. Actuators A 128 (2006) 109-115 2. Bharat Bhushan, Adhesion and Stiction: Mechanisms, measurement techniques, and method for reduction, J.Vac.Sci.Technol. B, Vol.21, No.6, Nov/Dec 2003 3. Komvopoulous, K. "Surface engineering and microtribology for microelectromechanical systems." Wear 200 (Dec, 1996): 305-327. 4. http://www.memsurface.com/stiction, date:1 March 2007 5. http://www.wikipedia.org date:1 March 2007