Characterization of materials involves measurement and analysis of different parameters namely:
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Physical - Dimension and Angles, Thickness, linewidth, Defects
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Chemical – Elemental composition and Dopant Concentration
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Electrical – Conductivity
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Surfaces – Structural, morphology, texture
Instruments used for subject magnification
- Optical microscopy
- Electron microscopy techniques (SEM, TEM)
- Scanning probe techniques (STM, AFM)
Optical Microscope
Components of an optical microscope
Ocular lens (eyepiece) (1) Objective turret or Revolver or Revolving nose piece (to hold multiple objective lenses) (2) Objective (3) Focus wheel to move the stage (4 – coarse adjustment, 5 – fine adjustment) Frame (6) Light source, a light or a mirror (7) Diaphragm and condenser lens (8) Stage (to hold the sample) (9)
Incident Light after passing through the sample can be analyzed by
- Transmission
- Reflection
A microscope should have a brightness and contrast of light from the object as well as high depth of field. Brightness refers to the intensity of light. In light microscope the brightness is related to the numerical aperture (NA) and magnification (M).
Working:
Two general categories for optical detection.
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Brightfield detection: examines the wafer surface with directly reflected light. With bright field detection, horizontal surfaces reflect most of the light while slanted or vertical surfaces reflect less.
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Darkfield detection: examines light scattered off defects located on the wafer surface. This is done by directing light to the wafer surface at a shallow angle through the outside of the optic’s objective body . This light impinges on the wafer surface and passes back up through the center of the optics. This action renders all flat surfaces black, while irregularities appear as bright lines. This fact makes darkfield detection useful for bringing out small defects on the wafer surface that might be difficult to see with brightfield detection.
Scanning Electron Microscopy (SEM)
What we can get in SEM?
Topography The surface features of an object or "how it looks", its texture; direct relation between these features and materials properties
Morphology The shape and size of the particles making up the object; direct relation between these structures and materials properties
Composition The elements and compounds that the object is composed of and the relative amounts of them; direct relationship between composition and materials properties
Crystallographic Information How the atoms are arranged in the object; direct relation between these arrangements and material properties
Secondary Electrons :
SEs are the products of inelastic scattering between primary electrons (beam) and loosely bound electrons of the specimen. They have an energy level of several keV. SEs can escape only from a volume near the specimen surface with a depth of 5–50 nm, even though they are generated in the whole pear-shaped zone.
Backscattered electrons :
BSEs are the products of elastic scattering, energy close to that of incident electrons. There is interaction between the incident beam and the nucleus of the specimen. Their high energy enables them to escape from a much deeper level in the interaction zone, from depths of about 50–300 nm. The lateral spatial resolution of an SEM image is affected by the size of the volume from where the signal electrons escape
Electron and sample interaction
Interaction Volume: volume inside the specimen in which interactions occur while interacting with an electron beam Depends on:
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Higher atomic number materials absorb or stop more electrons , smaller interaction volume
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higher acceleration voltages penetrate farther into the sample and generate a larger interaction volume
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the greater the angle (further from normal) the smaller the interaction volume
Electron guns in SEM
There are different types of elements used as the source of the incident beam
Comparison of the electron guns
Transmission Electron Microscopy
Transmitted electrons provide the information but poses limitation on sample thickness
Sample preparation: Since it is transmitted beam analysis the sample thickness should be small -~ 100 nm.
Type of Samples: Metal, ceramics, polymers, bio
Steps: 1. Pre thinning – reduce the size upto 0.1 mm by grinding
Steps: 2
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For metals – Electrolytic thinning by Jet polishing
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For ceramics – ion milling
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For soft materials (polymers and bio) Ultramicrotomy
Comparison of optical / SEM and TEM
Other Applications of microscopy instruments
Phase determination using TEM - Selected area electron diffraction (SAD or SAED)
Formed by placing an aperture in the imaging plane of the objective lens.
The combination of imaging and diffraction in the SAED mode makes it particularly useful for setting diffraction conditions for imaging in a TEM. It is also one of major techniques for materials phase identifications and orientation determinations.
Indexing of Electron Diffraction Pattern
“Selected Area Diffraction” (SAD) – selection of area using an aperture located below the sample holder
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Measure the diameter with reference to the center spot (transmitted beam)……from which radius ‘R’ calculated.
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Find out Lλ (which is camera constant)…. If camera constant is unknown use radius – ratio method….R1d1=R2d2
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Apply Lλ = Rd where d is the interplanar spacing, calculate d value for each ring.
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If you know the material compare with standard diffraction pattern to get the (hkl) values.
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From d spacing calculate lattice parameter.
Analysis of Chemical Information from EDX (in SEM /TEM)
EDX – an additional attachment tool in SEM/TEM termed as Wavelength and Energy Dispersive Analysis of X-rays (WDS/EDS)
There are four primary components of the EDS setup: the beam source; the X-ray detector; the pulse processor; and the analyzer A detector is used to convert X-ray energy into voltage signals; this information is sent to a pulse processor, which measures the signals and passes them onto an analyzer for data display and analysis.
The Lithium drifted Silicon (SiLi) detector The detector used in EDX is the Lithium drifted Silicon detector. This detector must be operated at liquid nitrogen temperatures. When an X-ray strikes the detector, it will generate a photoelectron within the body of the Si. As this photoelectron travels through the Si, it generates electron-hole pairs. The electrons and holes are attracted to opposite ends of the detector with the aid of a strong electric field. The size of the current pulse thus generated depends on the number of electron-hole pairs created, which in turn depends on the energy of the incoming X-ray. Thus, an X-ray spectrum can be acquired giving information on the elemental composition of the material under examination.
EDS Detector
Cross section of a typical Li drifted Si detector. X-rays create an e and hole pairs in the intrinsic region of the semiconductor, these charge carriers then migrate under the influence of an applied bias voltage
Why we need vacuum in electron microscopy?
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Vacuum provides allowance for the voltage difference between the cathode and the ground without generating an arc.
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to reduce the collision frequency of electrons with gas atoms to negligible levels
Typical vacuum for electron microscopy is in the order of 10-4 to 10-7 Pa. Depending on the pump used different ranges of pressure is maintained in the electron chamber
Rotary pumps > Atm to 10^ -4 Torr Diffusion pumps > 10^ -1 to 10^ -10 Torr Turbomolecular pumps > 10^ -2 to 10^ -10 Torr