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Optical methods for semiconductor characterization Guillaume von Gastrow.

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Presentation on theme: "Optical methods for semiconductor characterization Guillaume von Gastrow."— Presentation transcript:

1 Optical methods for semiconductor characterization Guillaume von Gastrow

2 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 phase e.g. interferometric spectroscopy Measurement of the electric field direction e.g. ellipsometry, Raman spectroscopy

3 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

4 Outline I- Observation of the sample – optical microscopy a. Fundamental notions b. Techniques II- Measurement of the physical characteristics III- Detection of defects and impurities

5 Optics notions  Focal point: for an optical system, point where the rays parallel to the optical axis converge (or seem to converge) Source:  Focal plane: plane perpendicular to the optical axis and containing the focal point

6 Interference phenomenon  Caused by interaction between several coherent light sources that have the same amplitude but a phase difference source:

7 I- Sample observation with optical microscopy a. Fundamental notions  Microscope performance is limited by interferences The angular position of the first minimum is given by  : demi-angle of the cone D: 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

8 Example: for a good numerical aperture i.e. NA=1, and =500 nm the minimum separation is s = 0,305  m I- Sample observation with optical microscopy a. Fundamental notions

9 Focus when the resulting image is at infinity (schema) Source: I- Sample observation with optical microscopy a. Main techniques 1. Optical microscope

10 Contrast is reduced by: -Dirty lenses (of course!) -Too powerful light Illumination can be controlled by the diaphragm aperture AdvantagesSimple, well-developed with many improvements DrawbacksResolution limited to 0,25 um I- Sample observation with optical microscopy a. Main techniques 1. Optical microscope

11 2. Confocal microscope I- Sample observation with optical microscopy a. 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 )

12 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 microscopy a. Main techniques Examples of confocal microscope pictures (Lasertech corporation)

13 3. Interferometric spectroscopy I- Sample observation with optical microscopy a. 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 I 0 : h(x,y) is the comparison of the sample height with a reference mirror

14 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 Implementation with a Linnik interference microscope

15 -Optical constants (refractive index, absorption constant) -Layer thickness -Linewidth -Crystallographic structure II- Physical parameters measurements

16 Measures the change of polarization of light reflected from a surface. Source: 1. Ellipsometry II- Physical parameters measurements

17 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

18  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 1. Ellipsometry II- Physical parameters measurements

19 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.

20  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 ABO 3 )

21 Example of a Raman spectrum after FTIR: intensity I in function of the polarization angle  and the frequency f (perovskite, DyScO 3 ) 2. Raman spectroscopy II- Physical parameters measurements

22 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)

23 -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

24  Applications Used for shallow-level impurities detection + deep-level if radiative recombination is possible  Principle Measurement 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

25 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)

26  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 D: particle diameter : laser wavelength K: relative dielectric constant of the particle Particle density detected by scanning the laser across the sample

27 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

28 Conclusion  Main advantages of optical characterizations: - Often easy to perform - Contactless - Non-destructive  Even basic methods have a lot of additional features


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