What is a Measurement Measurable Parameters

MEMS Metrology Metrology

Dr. Bruce K. Gale • What is metrology?
Fundamentals of Micromachining
– It is the science of weights and measures
What is a Measurement
• Refers primarily to the measurements of length,
• A measurement is an act of assigning a wetight, time, etc.
specific value to a physical variable
• Mensuration- A branch of applied geometry
• The physical variable becomes the
measured variable – It measure the area and volume of solids from
lengths and angles
• Measurements provide a basis for
judgements about • It also includes other engineering
measurements for the establishment of a
– Process information flat, plane reference surface
– Quality assurance
– Process control Measurable Parameters

• What do we want to • Pressure
measure? • Forces
• Stress
• Length or distance • Strain
• Mass • Friction
• Temperature • Resistance
• Elemental composition • Roughness
• Viscosity • Depth
• Diplacements or • Intensity
• etc.
distortions
• Time

Measurement Systems and Tools Components of a Measuring
System
• Measurement systems are important tools
for the quantification of the physical How Important are
variable Measurements?

• Measurement systems extend the abilities of • Measurement is the language of science
the human senses, while they can detect and • It helps us communicate about size, quantity,
recognize different degrees of physical
variables position, condition, time, etc.
• Simple measurement errors can cost a company
• For scientific and engineering measurement,
the selection of equipment, techniques and a contract, work, jobs, and lots of money
interpretation of the measured data are • Three areas to which the basic principles of
important
measurement can be applied
Importance of Metrology
– Communication of the measurement
• In human relationships, things must be – Act and application of the measurement
counted and measured – Codification of the measurement

• Metrology is an absolute necessity for
human development

– This necessity increased greatly with the advent
of the industrial age

• As society develops further, metrology must
also be refined further

Human Interaction in What is Microsystem Metrology
Measurements
• Measurement of physical dimensions of
• Almost always require the use of vision microsystems or structures that are at the
dimension scale below our ability to perceive
– Other senses not always good at “measurement” without the help of measurements systems or
instruments
• Measurements usually require tools
• Microsystems: IC, MEMS
– They rely on the visual process • Microstructures: Line widths, film thickness,

• What happens if object is too small to perceive? surface struture and roughness, step heights,
particle size, atomic composition, defect
– At what size do things become to small to measure? inspections, etc
– Handling and/or perception problems • Nanosystem metrology is emerging now!
– Limits of vision/ perception without aid or assistance
MEMS Measurement
Where Metrology at the Micro
Level is Used • Statistics and probability
• Statistical process control
• Precision engineering and measurements • Optical microscopy
• Micron and nano manufacturing • Scanning electron microscopy (SEM)
• Research and development • Atomic force microscopy (AFM)
• Calibration of instruments and standards • Scanning tunneling microscopy (STM)
• Near- field microscopy
• Laser/ White light interferometry
• Video Microscopy
• Surface Profiler
• Roughness Tester
• Ellipsometry

Statistics and Probability Statistical Process Control

• Statistical terms and • Bias and precision • Concept of variation • Population parameters
definitions errors
• Importance of SPC in • Control limits
• Infinite statistics • Error propagation production processes
• Finite statistics • Regression analysis • Subgroup average and
• Standard deviations • Least squares methods • Controlled and range
• Probability density • Linear polynomials uncontrolled variation
• Student’s t-Test • Three sigma limits
functions • Single and multiple • Common and special
• Confidence intervals causes – Six now!
• Uncertainty analysis measurement errors
• Error sources • Zero and higher order • Control charts

uncertainties • Statistical inference
• Correlation coefficient
• Running records

Optical Microscopy Numerical Aperture

• Light propagation and Snell’s law
• Mirrors, prisms, lenses and beam splitters
• Image formation, interference and diffraction
• Simple and compound microscopes
• Mirror and thin lens equations
• Resolving power and resolution
• Aberrations and corrections
• Depth of field and focus
• Measurement on the optical microscopic scale

MEMS Optical Microscopy Problems Scanning Electron Microscopy

• Used for observation, analysis, and measurement
• Can produce images over a wide range of

magnifications

– High magnification and great depth of field
– High resolution (down to 2.5 nm)

• Can provide morphological, compositional, and
physical information

• 3D appearance of image
• Greater degree of freedom

Near Field Scanning Optical NSOM
Microscopy
Images using NSOM
• Used to get below the limits of “far-field diffraction”

• High resolution down to 50 nm

Tip

SEM Components SEM Operation

• Electron gun assembly • Small area irradiated by electrons
• E-beam can be static or swept
– Stable source of primary electrons • Secondary and backscattered electrons detected

• Electromagnetic lenses and apertures Comparison of
Systems
– Focus electron beam

• Vacuum system

– Allows passage of electrons without
interference

• Electron collector, display, and recorder
• Specimen stage

– Goniometer stage

SEM Operation

• Current in the focused e-beam determines magnitude of
emitted signals

• Size of focused beam determines the resolution

Optical SEM

Transmission Electron Atomic Force Microscopy
Microscopy (TEM)
• Atomically
• Only for thin materials sharp tip
• Usually used to study
• Measures
crystals minute atomic
• Gives good information forces that
cause repulsion
on nanometer structure
• Can measure
AFM Image of Silicon individual
atoms
• Silicon (111) plane- ridges 0.38 nm high
Scanning
Tunneling
Microscopy

Scanning Tunneling Microscopy Roughness Step Tester (RST)

• Sharp tip • Surface characterization
• Tunneling current • Interference microscope
• Optical measurement of
or voltage
measured surface
• Current increases
rapidly on
approach to
surface
• Atomic resolution

Surface Texture Surface Profiler

• Definition • Large tip scans across surface
• Excellent for measuring height of objects
– Repetitive or random • Sidewalls are not accurate
deviation from nominal • Variation dependent on tip diameter
surface that forms the 3-D • Vertical accuracy in angstroms
topography of the surface

• Roughness

– Closely spaced irregularities

• Waviness

– Wider spaced irregularities
(tool marks)

• Form

– Does not contribute to
surface texture

Ellipsometry Interferometry

• Light is rotated as it passes through medium • Reflected light waves interfere with each other
• If dielectric constant is known, can be used to • Used to measure thickness of thin films or

measure film thickness deflections
• Interference occurs at different locations based on

wavelength

Thin Films CCD Cameras (Video Imaging)
Change Colors
• Charge Coupled Devices (CCD)

– Metal - insulator - semiconductor devices
(MIS)

– Incident light generates charge on gate
– Induced voltage causes charge separation

• CCD cameras are arrays of MIS
devices

– Can be 10 microns square
– Charge collected and moved sequentially

to edge of device
– Dyes can be applied to transistor to detect

colors