Abstract: A terahertz sub-surface scanner is introduced that utilizes reflection mode non-contact interrogation of surfaces and interior layers of composite substrates with resolution of ~1 nm. Quantitative measurements are done by implementing a modified Beer-Lambert’s law.

1. AF2J.4.pdf
CLEO:2013 Technical Digest © OSA 2013
Terahertz non-invasive sub-surface nano-scanner
Anis Rahman and Aunik K Rahman
Applied Research & Photonics, 470 Friendship Road, Ste. 10, Harrisburg, PA 17111
Email: a.rahman@arphotonics.net Phone: 717-623-8201
Abstract: A terahertz sub-surface scanner is introduced that utilizes reflection mode non-contact
interrogation of surfaces and interior layers of composite substrates with resolution of ~1 nm. Quantitative
measurements are done by implementing a modified Beer-Lambert’s law.
KEYWORDS: Spectroscopy: 300.6495 Spectroscopy, terahertz, Spectroscopy: 300.6470 Spectroscopy, semiconductors, Nonlinear
optics: 190.3970 Microparticle nonlinear optics.
Terahertz scanning can be used in different modes for different measurements. For wafer defect inspection the
machine may be used either in reflection mode or in transmission mode (for transparent substrates). A simultaneous
reflection and transmission measurement may also be done in some cases. The reflected beam intensity is
proportional to the physical properties of the specimen under test (SUT) such as the refractive index, density, surface
texture, etc. Ordinarily, the Beer-Lambert’s law is used to determine the concentration, C, of a solute in a solvent
from absorbance data: A = εlC, where l is the path length (thickness) and ε is the extinction coefficient (or molar
absorptivity). However, for a given wafer all material parameters may be assumed fixed because terahertz radiation
is non-ionizing and thus do not perturb the intrinsic properties. The reflectance in this case becomes a function of
different material properties on the substrate. That is, the reflectance, R, is proportional to the variation in material
conditions; thus, measurement of R(x) will yield the characteristics of the features (patterns) on the substrate. In
addition, if there is a hole on the substrate or in any of the layers, that will show in both reflected and in transmitted
intensity. Based on the above principle, a signature of a given imperfection may be established either in reflection
or in transmission. Any defects such as, inclusions, cracks, non-uniformity, or very small particulate material can be
detected and identified by this technique. Moreover, defect size may be estimated from a reconstructed 3-D scan.
All optical sources can examine surfaces by imaging or sensing of some kind. What distinguishes terahertz from
other methods is its ability to penetrate surfaces and be able to interrogate sub-surface layers and laminate materials.
For example, with ARP’s terahertz spectrometer (TeraSpectra) one can conduct transmission measurements to
calibrate a substrate for its thickness and/or defect layers. Alternatively, with ARP’s tearhertz scanning
reflectometer (TeraScanR) one can profile the concentration gradient of an analyte in to a substrate across its
thickness in a non-destructive fashion. Such capabilities are unique to terahertz technology. ARP’s terahertz
spectrometer operates over ~0.1 THz to ~35 THz and power level of <10 mW, CW. The low photon energy of Tray is not strong enough for breaking the bonds. T-rays are non-ionizing, therefore, is safe for human tissue; and
also for probing intrinsic properties of delicate materials. Therefore, all operations are non-invasive and nondestructive.
Terahertz portion of the electromagnetic spectrum (Fig. 1) provides some unique features that are not available from
other sources. Terahertz can penetrate most materials except metals; thus, it provides the opportunity to inspect not
only the surface but also the sub-surface interior layers of a multi-layered substrate. Unlike X-ray, terahertz is nonionizing; therefore, it does not perturb or damage delicate features which in some cases are only a few nanometers.
Yet, because of its very high sensitivity, terahertz can detect defects of nanometer size. The combination of
terahertz properties and a smart positioning system incorporated in the nano-scanner provides opportunity to
successfully inspect wafers at early stage of defect formation as well as for after-process device failure analysis.
Once the cause of defect formation is identified, appropriate measure may be taken to prevent its reoccurrences.
ARP’s terahertz scanner is designed to help exactly this situation. It deploys a non-contact measurement system
with an adjustable stand-off distance. The long axis is adjustable to accommodate required sample size; it can be up
to 250 mm long with a resolution of ~25 nm. However, an even longer arrangement is possible, but with slightly
lower resolution. Both y- and z-axes has a resolution of ~1 nm. The rotary axis has a resolution of ~4.1 µrad; as
such nano-scale features may be investigated. The rotary axis is important because adding a high resolution rotary

2. AF2J.4.pdf
CLEO:2013 Technical Digest © OSA 2013
axis enables examination of a wafer (or other sample) from different viewing angles. This is important because
cracks or other non-uniformities might NOT be along a straight line-of-sight. Thus an angular scan enables viewing
hidden features. Fig. 2 shows the resolution is ~1 nm and Fig. 3 shows the reproducibility of measurements. Details
of the method with exemplary data will be discussed.
In summary, the terahertz technology as implemented in the nano-scanner can help reducing the rejects of wafers
thus help improving the yield and output of the modern fabs.
Fig. 1. Position of terahertz is in between the microwave and infra-red (IR).
Fig. 2. Scan resolution is ~1 nm.
Fig. 3. Reproducibility is reasonably good.