test, new applications in industry and science are now possible
with the Protemics terahertz imager.
In the scientific sector, for example, researchers at the Dutch
Institute for Fundamental Energy Research (DIFFER; Eindhoven,
Netherlands) found that resonance frequencies of metamaterials
observed at far-field distances can differ significantly from the
ones monitored in the near-field range. 4 Their research showed
values of 0.62 THz at 1 µm distances vs. 0.85 THz in far-field
distances (around 24 cm).
For future sensing applications of such structures, it is therefore very important to have direct access to the near-field properties that determine the sensor/analyte interaction properties.
Also, coupling mechanisms between closely spaced resonant
structures—giving rise to electromagnetically induced transpar-
ency—can be monitored directly at the interacting structures. 5
In one example, it was demonstrated that electromagnetically
induced transparency in metamaterials can be enhanced to an
absolute value >80% by tuning the long-range coupling states
of periodic metamaterials to the individual resonance frequencies of the involved metamolecules.
Industrial wafer analysis
Besides scientific applications, industrial analysis is increasingly relevant for terahertz imaging systems. For example, the contact-free determination of material properties such as sheet resistance values (Rsh) or other charge-carrier-related properties
of semiconductor wafers and solar cells can be retrieved from
the transmitted terahertz signal by the application of analyt-
ic model descriptions. For example, the Tinkham formula is
commonly used for sheet resistance extraction from terahertz
transmission data obtained at thin (< 10 µm) conducting layers
on semi-isolating substrates. 6
So far, contact-free measurements of sheet resistance distri-
butions with up to micron-scale resolution are not possible over
full wafer-scale areas. Such capabilities are, however, required
to effectively optimize the fabrication processes of microstructures on solar cells through direct inspection in an early stage
of production instead of doing cumbersome tests on fully processed cell series afterwards.
In one example, the formation of high-efficiency cell contacts
by etching-paste-induced silicon-nitride (SiNx) layer removal is
of particular interest. 7 There, an etch paste is used to open the
SiNx. To find the optimal etching end-point, the opening of the
SiNx is monitored in dependence to the curing temperature of
the paste by terahertz near-field transmission imaging. The goal
is to fully remove the SiNx layer and leave the diffusion layer below the contacts mostly unaffected regarding sheet resistance. While the visual control can provide information about
the completeness of the SiNx removal process, it cannot give
information about the remaining quality of the diffusion layer.
The terahertz transmission images, however, can clearly re-
veal when the opening of the SiNx is completed and the etching
of the diffusion layer has started. Even small (still acceptable)