Nondestructive testing (NDT) and condition monitoring (CM) are vitally important in many branches of manufacturing because they make it possible to test products for quality and safety without causing damage. Many techniques have been developed for the purpose. Terahertz (THz) is a new addition to the toolbox of NDT and CM.

THz is an optical non-contact technique that can be implemented inline or offline. It employs non-ionizing far-infrared wavelengths between 100 and 1000 micron to sense material properties, detect faults and visualize structures. It is similar to X-ray in its operation, but unlike X-ray it is safe because it uses low-energy beams. The range of wavelengths is similar to ultrasound, but unlike ultrasound it is optical, non-contact and does not require immersion in a medium. THz can be employed to examine a wide variety of products. Most insulating materials are transparent or semi-transparent to THz and can be examined by THz beams in transmission or reflection, revealing their dielectric properties and internal structures.

Currently, the most common application of THz is to monitor coatings and multilayer structures, where it can reveal uniformity of multiple layers and layer thickness, layer adhesion, delamination or debonds, corrosion or damage under coatings and water ingress into or under coatings. THz systems have been installed on many automotive production lines to monitor the quality of paint, for example. Recently, THz has been deployed in electric battery production to monitor the uniformity and quality of electrode structures.

THz can be used to examine a wide variety of materials, such as polymers, adhesives, chemicals, food or pharmaceuticals. Because it reveals material properties, THz can detect density, crystallinity or polymerization, curing effects, compositional variations, additive content and porosity. By tracking variations in properties, it can reveal degradation, wear and moisture content. By mapping internal reflections off interfaces, THz can image defects, cracks, inclusions and air bubbles.

A particularly promising area of THz applications, where it is increasingly being adopted by industry, is in monitoring hydrocarbon fluids. Pure hydrocarbons are transparent to THz and it can detect compositional variations, the calorific value or octane number of fuels, a wide range of contaminants in fuels and lubricants, oxidation products and moisture content down to 0.01%.

Another area of THz applications that is being rapidly developed is in electronics manufacture, where it can detect faults such as circuit shorts or breaks, identify defective contacts, reveal hot spots, measure conductivity, image internal structures and measure response time with sub-picosecond resolution. THz can also monitor the properties of semiconductors and substrates, such as complex permittivity, complex conductivity and carrier mobility.

As with all techniques, THz has limitations that have to be taken into account. Penetration depth is determined by material absorption and varies between a few mm to a few cm. Since THz uses long-wavelength light, its spatial resolution is typically 0.5 mm or larger. To mitigate this problem, near-field probe techniques that increase resolution to a few micron have been developed and are used in particular by the electronics industry. Another problem that can arise is strong absorption by water, which severely limits THz applications for moist materials or in aqueous environments.

Current THz instruments are relatively bulky (typically around 0.5 m³) and expensive (typically around USD 100k), both of which constitute an obstacle to wider deployment in industry. Only high-value products where consistent quality is paramount merit THz sensors in their production lines. This is about to change, as manufacturers of THz instruments are developing a new generation of THz photonic integrated instruments, which will be compact and inexpensive. Demonstrator devices are already being showcased at technology exhibitions.

To summarize, THz is a safe, non-contact, nondestructive technique that can be employed inline or offline to monitor a broad range of products. As the technology develops, it is becoming increasingly versatile, accessible, affordable and user-friendly. The future of THz in NDT and CM is bright!