MEMS: Characterization Characterization of MEMS Devices Prasanna S. Gandhi Assistant Professor, Department of Mechanical Engineering, Indian Institute of Technology, Bombay,
Recap Characterization of MEMS Motivation Principles of optics Tools for optical characterization Microscope Ellipsometer Profilometer
Today s Class Scanning Probe Microscopy based tools: STM and AFM Methods for characterization of mechanical properties
Limitations of Microscope Q: is it possible to increase the magnification of microscope indefinitely and expect improved resolution?? Minimum resolution possible is comparable with wavelength of light
SPM: STM and AFM STM invented in early 80s by Binnig and Rohrer. Real limitations: only used to image conducting materials. Cannot distinguish between atoms of different elements within a compound material.
STM: Fundamentals
STM: Fundamentals Tip Cantilever Electron Tunneling Surface University of Southampton Surface Science Group Tunneling current at distance about 10A Two methods Constant current mode Constant height mode: faster Remarkable sensitivity: current being exponential function of distance (1A change order of magnitude change in current) Measures surface of constant tunneling probability Surface has small area oxidized?? Valid for conductors only
STM Image STM image of copper and nickel atoms http://spm.phy.bris.ac.uk/techniques/afm/
AFM: Fundamentals Force Sample Contact Repulsive Tip-sample separation Attractive Non-Contact Force of interaction between molecules Tip <100A in dia Scanning of sample or tip to generate image Van der Waals forces between tip and sample Contact: few A, noncontact: 10-100A Force balance in contact regime? Additional force: Capillary force + cantilever force = repulsive VW force (10-6 -10-8 N) Detection using photodiodes
AFM: Operation Contact Mode Constant-height Fast speeds Atomic scale images Constant-force: cantilever deflection used as feedback to adjust z to maintain deflection constant Speed of scanning is limited Scan path Sample Non-Contact mode / tapping mode
AFM: Operation Non-Contact mode / tapping mode Vibration of AFM cantilever near surface of a sample Total force: 10-12 N very small Stiffer cantilevers necessary Operation near resonance frequency (typically 100-400KHz), amplitude 10-100A Change in the resonance frequency during scanning of sample Control can be used to keep resonance amplitude or freq constant Soft samples can be probed in this mode
AFM: Operation Other modes MFM: magnetic force microscopy LFM: lateral force microscopy EFM: Electrostatic force microscopy TSM: Thermal scanning microscopy NSOM: Near field scanning optical microscopy Nanolithography
Atomic Force Microscope Actual system details Multi-mode nanoscope from Digital Instruments: Physics Dept., IIT Bombay
Atomic Force Microscope The SPM head All figures of actual system are taken from Multimode SPM installation manual, RevB, Digital Instruments, 2004.
Atomic Force Microscope
Atomic Force Microscope
Atomic Force Microscope Application to MEMS Measurement of MEMS cantilever stiffness using AFM BioMEMS sensor characterization (ongoing activity) Nanoindentation using diamond tip Thin film surface characterization
AFM Image Kriptan- polymer surface characteristics using AFM
Application of techniques Characterization of Mechanical Properties Properties: E, ν, internal stress etc. Various Techniques Bending test Cantilever Beam Bulge test Resonance method M-Test Nanoindentation
Bending Test Cantilever k = 3 Ebt 4 l ( 1 ν 2 ) 3 k is the stiffness, E is the elastic modulus, b is the cantilever width, v is Poisson s ratio, t is thickness, and l is the length of cantilever at the point of contact,
Bending Test Fixed-fixed Beam = F = k bending z + k stress z + k stretching z 3 4 Ewπ t 6L 3 3 z bending, stress, and stretching components: Small loads: - bending and stress Large loads: - Stretching 2 wσ 0π t + z + 2L 4 Ewπ t z 3 8L E is the elastic modulus, b is the cantilever width, v is Poisson s ratio, t is thickness, and l is the length of cantilever at the point of contact, 3
Bulge Test Pressure on circular membrane p = 4tσ r 2 0 h + 8t 3r 4 E h 1 ν 3
Resonance method Vibrating cantilever f 0i = 2 λi t 4πl 2 E 3ρ Where E, ρ, l and t are the Young s modulus, density, length and thickness of the cantilever. λi is the eigen value, where i is an integer that describes the resonance mode number; for the first mode λ =1.875 1 2
M-Test K V eff pi = = 27 8K eff g 1 + γ 3 3 0 g ω 0 ε 0 n S 2 2 1 L 1 + kl 2 sinh kl { cosh } 2 kl 2 12S ~ 3 k =, S = σtg 0, B = B ~ Et 3 g Set of cantilever, fixed-fixed beam, circular diaphragm, fabricated on substrate: actuated by electrostatic pull Characterization is based on pull in voltage No necessity of displacement measurement 3 0
Nanoindentation: AFM Additional attachment to AFM r S = dp dh = 2 E π r A 2 2 ( 1 ν ) ( 1 ) 1 ν i = + E E E i
Conclusions Various optical principles Characterization tools Microscope Ellipsometer Profilometer Various methods of characterization of mechanical properties
Next class Polytec Laser Doppler Vibrometer [2]
Atomic Force Microscope Laser Alignment Crucial issues -Alignment -Calibration