Optical methods for semiconductor characterization Guillaume von Gastrow.

Optical methods for semiconductor characterization

Guillaume von Gastrow

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


1. Optical microscope

2. Confocal microscope


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


Examples of confocal microscope pictures (Lasertech corporation)

3. Interferometric spectroscopy


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


The characteristics of the sample are calculated from the amplitude and phase change and of the electric field component.

1. Ellipsometry


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


1. Ellipsometry


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


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)





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


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


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


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


Advantages Very small particles can detected, even « Drawbacks Detection limited by the surface roughness

Noise due to surface interference

Conclusion

Main advantages of optical characterizations:

- Often easy to perform Contactless Non-destructive

Even basic methods have a lot of additional features

Optical methods for semiconductor characterization Guillaume von Gastrow.

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