SEM vs TEM: Electron Microscopes in Semiconductor Analysis

Both microscopes use a beam of electrons instead of light, but they look at samples in different ways. A scanning electron microscope (SEM) scans a focused beam over the surface and builds an image from the electrons that come back, so it shows the shape of the surface. A transmission electron microscope (TEM) sends electrons through an extremely thin slice, so it can show the inside of the structure down to individual atoms. SEM is fast and easy; TEM is far more powerful but slow and demanding.

AspectSEMTEM
PrincipleA focused beam scans the surface; secondary or backscattered electrons form the imageElectrons pass through a very thin sample; transmitted and diffracted electrons form the image
What it showsSurface shape and composition contrastInternal structure, crystal lattice and atomic-scale interfaces
ResolutionAbout 1 nm at best; typically a few nanometers or moreBelow 0.1 nm: individual atoms can be seen
Sample preparationLittle: bulk samples can be used (insulators need a conductive coating)Very demanding: the sample must be thinner than about 100 nm, often cut by focused ion beam (FIB)
Beam energyAbout 0.5 to 30 kVAbout 80 to 300 kV
SpeedFast: minutes per sampleSlow: hours, including sample preparation
CostLowerHigher
Typical usesLine-width measurement (CD-SEM), defect review, cross-section views of devicesGate-oxide thickness, thin-film stacks, crystal defects and interface structure

Figures are typical orders of magnitude and vary by process generation and product.

When to use which

SEM
For routine inspection and measurement, where speed matters, such as checking patterns and finding defects.
TEM
When you need to see inside a structure at the atomic scale, such as the thickness of a gate dielectric a few atoms thick.
Together
In failure analysis, SEM and FIB find and expose the problem spot, and a thin slice is then cut out for TEM.

FAQ

Which has higher resolution, SEM or TEM?

TEM. It resolves below 0.1 nm, so individual atoms can be seen, while an SEM resolves about 1 nm at best.

Why does TEM need such a thin sample?

Electrons must pass through the sample, so it must be thinner than about 100 nm. Preparing it, often with a focused ion beam, takes hours.

What does a CD-SEM measure?

A critical-dimension SEM measures the line widths and hole sizes of patterns on wafers. It is a key tool for process control in lithography and etching.

Why are insulating samples coated before SEM?

Insulators build up charge under the electron beam, which distorts the image. A thin conductive coating, or imaging at low voltage, prevents this.

Check your understanding

Which correctly describes the principle of the scanning electron microscope (SEM)?

  1. AA finely focused electron beam is scanned over the sample surface, and secondary electrons and other emissions are detected to observe surface shape
  2. BCrystal structure is examined from X-ray diffraction
  3. CFilm thickness is measured from light interference
Show answer and explanation

Answer: A. A finely focused electron beam is scanned over the sample surface, and secondary electrons and other emissions are detected to observe surface shape

An SEM scans a finely focused electron beam over the sample and forms images from the emitted secondary and backscattered electrons. Low-energy secondary electrons escape only from very shallow depths, so they reveal topography, while backscattered electrons increase with heavier elements and show compositional contrast. Resolution is around 1 nm. Fabs use CD-SEMs, operated at low accelerating voltage, to measure linewidths for process control. X-ray diffraction and optical interference are different techniques.

Which correctly describes the sample preparation needed to observe a sample with a transmission electron microscope (TEM)?

  1. AStack samples to a thickness of several cm
  2. BThin the sample to about 100 nm or less so electrons can pass through it
  3. CDissolve the sample in liquid
Show answer and explanation

Answer: B. Thin the sample to about 100 nm or less so electrons can pass through it

A TEM forms images from electrons transmitted through the sample, so the specimen must be thinned to about 100 nm or less for the beam to pass. For semiconductor devices, the lift-out method is common: an FIB mills around the region of interest, extracts a thin lamella and finishes it. Aberration-corrected TEMs resolve below 0.1 nm, showing individual atomic columns. Thick stacks or dissolved samples cannot transmit electrons.

What can be determined with EDX (energy-dispersive X-ray spectroscopy), used together with SEM and TEM?

  1. ASurface roughness
  2. BThe kinds and amounts of elements in the sample
  3. CInternal stress in films
Show answer and explanation

Answer: B. The kinds and amounts of elements in the sample

When an electron beam ejects an inner-shell electron, an outer electron dropping into the vacancy emits a characteristic X-ray whose energy is unique to the element. EDX analyzes these energies to identify elements and their amounts. Because it reveals composition right where the microscope is looking, it is widely used to analyze particles and failure sites. Its detection limit is about 0.1 wt%, so trace impurities require techniques such as SIMS.

  1. What are the main uses of focused ion beam (FIB) systems?
  2. When observing insulating samples with an SEM, which is a correct way to prevent charging (charge-up)?
  3. What is mainly measured with an atomic force microscope (AFM)?

Practice Metrology and analysis questions

Key terms explained

SEM (scanning electron microscope)
A microscope that scans a finely focused electron beam over a sample and detects secondary electrons and other signals to image the surface. It resolves features of a few nanometers, far beyond optical microscopes.
TEM (transmission electron microscope)
A microscope that observes thinned samples by passing an electron beam through them. With atomic-level resolution, it reveals cross-sectional structures and crystal defects.
FIB (focused ion beam)
An instrument that mills or deposits material with a finely focused ion beam. It is used for cross-sectioning tiny areas, preparing thin TEM samples and editing circuits.
AFM (atomic force microscope)
A microscope that traces a sharp tip over a sample and measures fine surface topography from the forces between tip and surface. It is used, for example, to evaluate surface roughness.

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