MEMS: Characterization Characterization of MEMS Devices Prasanna S. Gandhi Assistant Professor, Department of Mechanical Engineering, Indian Institute of Technology, Bombay,
Recap Fabrication of MEMS Conventional VLSI fabrication Nonconventional methods Design and analysis of MEMS Characterization of MEMS
Today s Class Why characterization?? Why optics?? Principles of optics useful in characterization Tools for optical characterization Profilometer Microscope Methods for characterization of mechanical properties SPM based tools: STM and AFM
Why Characterization? Material properties change at micro-scale, different from bulk properties due to grain boundary effect Successful design/manufacturing of MEMS devices need reliable knowledge of MEMS material properties Verification of design and validation of models proposed Calibration of devices and signals Electronic analysis: noise vs signal Research various new effects: example Biosensor devices
Why Optics for Characterization? Noninvasive technique Does not disturb sensitive MEMS device Very high resolutions possible Higher measurement range possible Several optical phenomenon can be made use of
Principles of Optics Wave nature of light Interference Wave division Amplitude division Diffraction + Diffr. grating Moire interference Holography
Principles of Optics Interference Wave division Amplitude division Beam splitter Young s double slit Reference mirror Michaelsons Interferometer Analysis??
Principles of Optics Interference Test device Mach-Zehnder Interferometer Used for laser-doppler vibrometer
Polarization Concept of polarization of light
Principles of Optics Interference Source Partially Reflecting Lens Mirrors Febry-Parot Interferometer Screen Another method for interference
Principles of Optics Diffraction grating Source Diffraction Fringes Diffraction Grating Diffraction Grating Fringes
Principles of Optics Moire Fringes Specimen Grating Fringes Master Grating Rotational Mismatch Translational Mismatch
Profilometer Profilometer principle A B D C Laser-photodetector combination As the scanning of sample is done the laser spot moves on the photodetector (PSD) because of bending of cantilever over asperities The movement results in differential voltage output from the PSD
Profilometer Another technology Sensor Spot size [µm] 1,5 Camera Integrated in-axis camera Vertical resolution [µm] 0,020 Field of view [mm] 0,6x0, 8 Measurement frequency [Hz] 10,000 copyright Solarius Development Inc. 2003-04 Stand off [mm] Linearity [%] 2 or 5 <0,08 Laser diode Wavelength [nm] Class I 630
Profilometer Another technology Sensor Camera copyright Solarius Development Inc. 2003-04 Spot size [µm] Vertical resolution [µm] 2 0,1 Integrated off-axis camera Magnification 200x Home Technology Products Applications Customer Support Company Contact All contents copyright Solarius Development Inc. 2003-04 Measurement frequency [Hz] Stand off [mm] Linearity [%] 1400 5 ±0.5 Laser Wavelength [nm] Class II 670
Microscope for Measurement of Dimensions Grating used in CD ROM Taking image on CCD camera and processing with precalibration for measurement of MEMS device dimensions Various types of microscopes
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.
Atomic Force Microscope
AFM Image Kriptan- polymer surface characteristics using AFM
Conclusions Various optical principles Characterization tools Microscope Ellipsometer Profilometer Various methods of characterization of mechanical properties
Grating used in CD ROM Fundamentals of Ellipsometry
Fundamentals of Ellipsometry Change in polarization properties after reflection
Ellipsometer Laser Detector P Q θ Surface A An ellipsometer measures the changes in the polarization state of light when it is reflected from a sample. If the sample undergoes a change, for example a thin film on the surface changes its thickness, then its reflection properties will also change. Measuring these changes in the reflection properties can allow us to deduce the actual change in the film's thickness.
Ellipsometer: Advantages Non destructive character, High sensitivity due to the measurement of the phase of the reflected light, Large measurement range (from fractions of monolayers to micrometers ), The possibilities to control in real time complex processes.
Next class AFM technique and details of measurment
Next class Polytec Laser Doppler Vibrometer [2]
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
Profilometer Profilometer principle A B D C Laser-photodetector combination As the scanning of sample is done the laser spot moves on the photodetector (PSD) The movement results in differential voltage output from the PSD