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Layer thickness ans crytalline standards

Working group 5.13

methodes of layer thickness measurements

The aim is the improvement of the measuring technique and traceability of layer thickness measurements to a dimensional quantity (length, area). Therefore different devices and preperation techniques are used.

Available measurement methodes



Layer thickness measurements on cross-sections with the scanning electron microscope

Layer thickness measurements on cross-sections require a plane of section through the layers of interest which allows the material transitions to be detected. The layer thickness results from the spacing of material boundaries in the recorded material profile.
Before the measurement is performed, the cross-sections of the objects to be measured must be prepared. As the measurement objects are destructed during this process, comparable reference objects should be available.

 

Figure 1: View at measurementlokation for thickness measurement

Measurement principle
Inside the chamber of the scanning electron microscope there is an uniaxial piezo-driven flexure stage equipped with a laser interferometer which determines the shifting length. The images of the cross sections are generated by the combination of an object scan of the sample vertical to the material boundaries and the electron beam scan parallel to the material boundaries. The obtained image has measured line distances by laserinterferometer (traceable).

Figure 2: Principle of the combined electron beam and object scan

 


Technical data:

  • REM: JEOL 6300F (field emission cathode) with detectors SE (Everhardt Thornley), BE (Robinson), EDX (SiLi)
  • Resolution SE 1.5 nm; BE 3 nm (30 kV)
  • Positioning stage: uniaxial piezo-driven solid stage translation stage, displacement length 50 µm
  • Displacement measuring system: Interferometer, resolution 1.0 nm
  • U(k=2) = 20 nm + 10-2 · h, for h = 50 nm to 1 µm
  • U(k=2) = 25 nm + 5 · 10-3 · h, for h = 1 µm to 40 µm

Ellipsometric layer thickness measurements

Ellipsometry is a non-destructive, optical measurement method for determining the dielectric properties of thin film systems by measuring the polarization change of electromagnetic waves. Ellipsometry is a very precise and fast measuring method. It offers unprecedented possibilities for metrology of nanostructures.

Figure 3: Principle of ellipsometry

 

Instrumentation
Technical data
•    Spectral ellipsometer manufactured by Semilab, type GES-5E
•    Rotating polarizer
•    Microspot optics (automatic)
•    Light source : 75 W Xenon lamp
•    Variable angle of incidence/outfall 40° - 90°.
•    Spectral bandwidth : 190 nm - 1000 nm
•    Measuring spot : 1 - 5 mm (parallel beam), 200 µm (microspot)
•   Detector system : grating spectrometer with PMT (high resolution) CCD spectrometer (fast data acquisition)
•    Measurement range: 5 nm to 1000 nm (SiO2), U(k=2) = 1 nm to 5 nm

 

Figure 4: Measuring setup for ellipsometric coating thickness measurement

 

Major applications
The spectral ellipsometer for thickness measurements of optical coatings is located in the clean room center of PTB. Two working areas are addressed:

  1. The calibration of SiO2 layers on flat Si substrates as layer thickness standard in the nanometer range.
  2. The surface characterization on silicon spheres of the Avogadro project, which is dedicated to a new option for the possible redefinition of the SI unit "kilogram".

 

 


Electromagnetic layer thickness measurements

Non-destructive measurements of the thickness of a non-ferromagnetic layer on ferromagnetic substrate (nFe // Fe) and of a non-conductive layer on conductive non-ferromagnetic substrate (nL // LnFe) are possible by touching the layer surface with a probe. For calibration of the measuring device, only an uncoated substrate sample of identical surface quality is required. The principle of the method is that the layer is replaced by a definitely generated air gap between measuring probe and substrate sample.

 

 

Figure 5: Measurement system for electromagnetic layer thickness measurement

Measuring procedure Measurement object Measuring range
magnetic induction nFe // Fe 10 µm ... 2000 µm
eddy current nL // LnFe 10 µm ... 1200 µm

 

The achievable measurement uncertainty depends on the layer thickness and on the material combination. For uniformly thick layers and favourable conditions, values of < 2% of the layer thickness can be achieved for the expanded uncertainty (k=2).

 

 


Layer thickness measurements by topographic methods

In comparison to ellipsometric methods, the topographic methods need coated and non-coated regions of the substrate which are in neighborhood and which may be scanned together.
The coating thickness may be calculated from the difference in the height between the coated and the non-coated parts of the surface.

Often the measuring objects have to be prepared by a mechanical process (removing of parts of the coating Opens internal link in current windowPreparation of specimens).
Because the measuring objects will be destroyed during the procedure, it is necessary to have similar reference objects.

The measuring methods to use (Interference microscope, Mechanical profiler, Scanning probe microscope) depend on the specimen's structure (mechanical and optical properties as well as the needed uncertainty. For details Opens internal link in current windowPossibilities of measuring and calibration.

 

 


Thickness measurements of foils

The thickness of foils is directly measured by mechanical probing with a calibrated inductive length measurement system. It results as vertical spacing between the two foil surfaces. The geometrical conditions and the measuring force are adjusted in accordance with the intended use of the foils (statements of customer).

 


Figure 6: Measurement setup for thickness measurements of foils

When uniformly thick foils are calibrated, the following measurement uncertainties can be achieved:

film thickness h

expanded measurement uncertainty U (k=2)
10 µm ... 200 µm 0,1 µm
> 200 µm ... 400 µm 0,2 µm
> 400 µm ... 2000 µm

0,6 µm


X-ray reflectometry (XRR)

X-ray reflectometry (XRR) is a non-destructive method  measuring thickness of ultrathin layers. It uses the reflection of X-rays at flat angles of incidence (up to approx. 5 °) on thin-film systems to calculate the layer thickness from the measured interference pattern.

Figure 7: Principle sketch of the XRR

 

For single layers, the layer thickness can be calculated directly from the distance of the intensity maxima (or minima), from the measured angle and the wavelength of the X-rays. In this case, standard uncertainties in the sub-nanometer range can be achieved for layers of 5 nm or more.

Figure 8: Example of a reflectogram for a platinum layer approx. 50 nm thick.

 

In multilayer systems, the measured reflectogram is fitted with a simulation program and the measured layer thickness is determined from the fit parameters. The influence of the optical constants on the measurement uncertainty is negligible. For single layer systems it is not necessary to know the optical constants of the material.
Advantages of XRR

  • Non-destructive
  • low measurement uncertainty
  • negligible influence of material parameters

Limitations of XRR
•   limited measuring range (for laboratory sources max. 200 nm, depending on material density)



Preparation of samples for topographic measurements and measurements on cross-sections

Layer thickness measurements can often be performed only after the samples have been adequately prepared.
Preparation of cross-sections:

  • Sample preparation by separation, embedding, grinding. Occurring disruptions and material contaminations on the grinding face influence the result of the layer thickness measurement
  • Post processing of the cross-sections by ionic bombardment observed under the electron microscope

 


Figure 9: Principle of ion beam processing


Figure 10: View of the ion beam preparation facility

 

 

 

Selective layer removal

  • Locally restricted chemical or electrochemical layer detachment. Layer residuals and substrate ablation at the place of detachment impede the layer thickness measurement
  • Material-specific resolutions and process parameters
  • Experience gained with copper and nickel layers on steel substrate

 

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 Figure 11: Prinzip of electrochemical layer detachment