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Optical measurements: contactless methods, no preparation
Three categories
I. Introduction
Photometric
Interference
Polarization
Measurement of the intensity e.g. optical microscope, reflection measurements
Measurement of the phasee.g. interferometric spectroscopy
Measurement of the electric field directione.g. ellipsometry, Raman spectroscopy
Parameters measured by optical methods
Measured characteristics
Physical characteristics
- Optical constants- Thickness- Linewidth- Crystallographic
structure
Sample aspect- Thickness- Shape- Cross section- Surface aspect
Defects and impurities
- deep/shallow level impurities
- particles
e.g. photoluminescence, elastic light scattering
e.g. optical microscope, interference contrast microscope, ellipsometry
e.g. transmittance spectroscopy, Raman spectroscopy, ellipsometry
Outline
I- Observation of the sample – optical microscopya. Fundamental notionsb. Techniques
II- Measurement of the physical characteristics
III- Detection of defects and impurities
Optics notions
Focal point: for an optical system, point where the rays parallel to the optical axis converge (or seem to converge)
Source: http://www.bradleypatten.co.nz
Focal plane: plane perpendicular to the optical axis and containing the focal point
Interference phenomenon
Caused by interaction between several coherent light sources that have the same amplitude but a phase difference
source: http://gilbert.gastebois.pagesperso-orange.fr
o Optical path difference
o Constructive interference if Destructive interference if with m natural integer
I- Sample observation with optical microscopya. Fundamental notions
Microscope performance is limited by interferences
The angular position of the first minimum is given by
: demi-angle of the coneD: aperture diameter
Rayleigh principle: two objects can be distinguished if the central maximum of one coincides with the first minimum of the other
Maximum resolution achievable
d
22,1sin
Example: for a good numerical aperture i.e. NA=1, and =500 nmthe minimum separation is s = 0,305 m
I- Sample observation with optical microscopya. Fundamental notions
Focus when the resulting image is at infinity (schema)
Source: http://ressources.univ-lemans.fr
I- Sample observation with optical microscopya. Main techniques
1. Optical microscope
Contrast is reduced by:- Dirty lenses (of course!)- Too powerful light
Illumination can be controlled by the diaphragm aperture
Advantages Simple, well-developed with many improvements
Drawbacks Resolution limited to 0,25 um
I- Sample observation with optical microscopya. Main techniques
1. Optical microscope
2. Confocal microscope
I- Sample observation with optical microscopya. Main techniques
Principle: 3D picture of an object thanks to different focalizations. One pixel at a time, light outside of the focal plan is eliminated.
Better contrast
Resolution:
Applications: scanning trough the sample (e.g. different heights in integrated circuits)
Advantages Better resolution and better contrast than optical microscope
Drawbacks Signal-to-noise ratio reduced by too small pinhole sizes
2. Confocal microscope
I- Sample observation with optical microscopya. Main techniques
Examples of confocal microscope pictures (Lasertech corporation)
3. Interferometric spectroscopy
I- Sample observation with optical microscopya. Main techniques
Application
Determine vertical and horizontal features of a sample.
Principle (phase-shift interferometry)
For a monochromatic wave intensity after interference of two waves of intensity I0:
yxhKII ,4
cos10
h(x,y) is the comparison of the sample height with a reference mirror
Advantages
Resolution• x, y:
• z: 1nm
Drawbacks• Height ambiguity every /4• Error induced when two points of the surface are not in focus at the same time
NA
5,0
Implementation with a Linnik interference microscope
- Optical constants (refractive index, absorption constant)
- Layer thickness
- Linewidth
- Crystallographic structure
II- Physical parameters measurements
Measures the change of polarization of light reflected from a surface.
Source: http://jawoollam.com
1. Ellipsometry
II- Physical parameters measurements
The characteristics of the sample are calculated from the amplitude and phase change and of the electric field component.
1. Ellipsometry
II- Physical parameters measurements
Principle: Case of rotating analyzer ellipsometry
Light intensity at the detector:
Fit of the curve for and
Calculate and
Calculate n and k
Applications
- Measurement of optical constants- Film thickness
2a 2b
2arcosh2
1a
22
2
1arcosh
a
b
1. Ellipsometry
II- Physical parameters measurements
1. Ellipsometry
II- Physical parameters measurements
Advantages Contactless method (in-situ measurements), widespread for thickness measurements Variable wavelength
Drawbacks Interferences cyclic thickness values. Need to have a guess on the thickness value.
Principle
Activation of the vibration modes of a crystal by polarized light
The wavelength of the reemitted photons shifts
Analysis of the polarization changes of the reflected light after FTIR (Fourier Transform Infrared Spectroscopy)
Applications
- phase transition measurements- crystallinity tests (structure, orientation)stress measurements also impurities detection
2. Raman spectroscopy
II- Physical parameters measurements
Examples of vibration modes (perovskite ABO3)
Example of a Raman spectrum after FTIR: intensity I in function of the polarization angle and the frequency f (perovskite, DyScO3)
2. Raman spectroscopy
II- Physical parameters measurements
2. Raman spectroscopy
II- Physical parameters measurements
Advantages Able to characterize any kind of crystalline material, non-destructive method
Drawbacks Limitations caused by interferences due to fluorescence (sample or impurities)
- Semiconductor impurities (deep and shallow level)
- surface particles 1/3 of the smallest circuit dimension (eg gate thickness) is already detrimental
II- Defects and impurities
Applications
Used for shallow-level impurities detection + deep-level if radiative recombination is possible
PrincipleMeasurement of the internal efficiency, related to radiative electron/hole pairs
recombination
1. Photoluminescence
II- Defects and impurities
Fig. : Different types of recombination
(a) Band-to-band(b) Free exciton(c) Bound exciton(d) Free e/hole pair(e) Acceptor donor
Advantages Very high sensitivity
Drawbacks Low temperature measurement No difference bulk/surface
1. Photoluminescence
II- Defects and impurities
Bound exciton recombination dominates over free exciton recombination for less pure material.
PL of a GaN structure(http://www.ioffe.rssi.ru)
Principle
Detection of the light scattering by surface particles in all directions.The detectors are placed at various locations.
The scattered light is proportional to the optical scattering cross-section (for D « ):
2. Elastic light scattering
II- Defects and impurities
2
4
64
2
1
18
K
KD
D: particle diameter: laser wavelengthK: relative dielectric constant of the particle
Particle density detected by scanning the laser across the sample
2. Elastic light scattering
II- Defects and impurities
Advantages Very small particles can detected, even « Drawbacks Detection limited by the surface roughness
Noise due to surface interference