Lecture 14: Stiction in MEMS ME 645: MEMS: ME 645: MEMS: Design Fabrication Design Fabrication Design, Fabrication Design, Fabrication and Characterization and Characterization P.S. Gandhi P.S. Gandhi Mechanical Engineering Mechanical Engineering IIT Bombay IIT Bombay Acknowledgments: G.M.VINOD & R.GIRISH PRASANNA S GANDHI gandhi@me.iitb.ac.in PRASANNA S GANDHI gandhi@me.iitb.ac.in Contents Contents Introduction I t I t Introduction d d ti ti � � Friction Vs Stiction Friction Vs 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 Reduction Stiction Reduction Methods Methods � 1
I ntroduction I ntroduction Why Surface Forces Dominate at Micro Level? Why Surface Forces Dominate at Micro Level? Fs: Surface Forces, Fs: Surface Forces, proportional to L proportional to L 2 2 2 Fb Fb: Body Forces, : Body Forces, proportional to L proportional to L 3 3 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 /r 2 Fb Fb’ = ’ = Fb Fb /r /r 3 Fs’/Fb Fs’/ 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: St Stiction : St atic atic Fr iction iction Fr • The threshold force requirement to overcome static cohesion/ adhesion between two surfaces • Surface adhesion forces are higher than mechanical restoring hi h th h i l t i forces at microscale 2
Comparison Comparison Friction Friction Friction Friction Stiction Stiction Stiction Stiction � Occurs even Occurs even without without a pre a pre- - Occurs between two objects Occurs between two objects � existing force on the members existing force on the members in contact with each other in contact with each other with a normal force Fn with a normal force Fn between them. between them. F f = μ f Fn Stiction force remains even Stiction force remains even � If the normal force of If the normal force of � after the initial force that after the initial force that friction is removed the force friction is removed the force brought them in contact is brought them in contact is of friction will be gone of friction will be gone removed removed � Proportional to the area of Proportional to the area of contact contact Independent of area of Independent of area of � contact contact � Release Stiction Liquid Film � Surfaces locked by the presence of thin film of liquid remaining after drying process � In use Stiction In Use Contact � Occurs when Micro Structures come into contact during operation (may not be because of liquid) 3
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 Capillary Force Capillary Force F F σ σ= Surface tension force F = Spring force t θ A = Wetted area Internal fluid pressure due to surface tension, P = (2 σ Cos θ )/ t Spring force = Internal fluid pressure x Area of plate F = ((2 σ Cos θ ) /t )x A 2 σ A Cos θ F = t 4
Hydrogen Bridging Hydrogen Bridging Hydrophilic Silicon Surfaces absorb water to a small depth Hydrophilic Silicon Surfaces absorb water to a small depth y y p p 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 g j 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 – – � Є A V 2 F = 2 h 2 2 h 2 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. 5
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 g p p � or grains of polycrystalline material. or 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] A Fv = d ≤ 30nm 6 π d 3 B d> 30nm Fv = d 4 A , B : Hamaker Constants for the material Comparision Comparision of Forces of Forces Causing Stiction Causing Stiction Figure by MIT OCW. After Komvopoulous, K. "Surface engineering and microtribology for microelectromechanical systems." Wear 200 (Dec, 1996): 305- 327. 6
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 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 Ref: http://www.memsurface.com/stiction Stiction Stiction Reduction Reduction Methods Methods � Choosing a suitable drying process to avoid solid liquid interface g y g p q � Sublimation Drying Solid Liquid � Supercritical Drying Pressure Vapour Temperature 7
References References 1 1. Dongmin Wu Nicholas Fang Cheng Sun Xiang Zhang Stiction 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 8
Recommend
More recommend
