Terahertz Technology: Poised for Manufacturing

Thanks to a range of developments in technology, systems based on terahertz technology are poised to enter and create significant new markets within the decade. A recently published market study by Thintri, Inc. highlights significant commercial opportunities in terahertz technology this decade. Of the many potential applications of terahertz radiation, manufacturing is potentially the most promising.

The terahertz portion of the electromagnetic spectrum is vaguely defined but is basically the band between infrared and microwave radiation, usually considered to run from 300 GHz to perhaps 10 THz, overlapping those bands commonly referred to as submillimeter and far infrared.

Terahertz radiation is a critical concern in astronomy, given that approximately one half the total luminosity of the universe and 98% of the photons emitted in the history of the universe lie in the terahertz portion of the spectrum, and that terahertz waves are not scattered by gas clouds in space.

Terahertz waves are reflected by metallic surfaces and absorbed by water, both of which remain opaque to terahertz signals. However, most other materials are transparent to terahertz radiation, to varying degrees. Terahertz systems can provide both images and spectroscopic data, (possibly in the same measurement), and ranging data that can measure coating or layer thicknesses, even in structures of many layers.

A number of technical breakthroughs in photonics, electronics and nanotechnology have occurred since the early 1990s which have brought terahertz technology within striking distance of significant commercial markets like security, communications, nondestructive evaluation, medicine and electronics. Bulk and ease of use have been longstanding issues with terahertz technology, but recently developed systems are as easy to use as an oscilloscope, and some are so small and robust that they can be delivered through the mail.

While development continues on components, attention is shifting to development of applications that are now ready to take advantage of the extraordinary versatility of the terahertz band. Indeed, application and market development are now the primary hurdles in the way of creation of significant markets for terahertz systems in such promising applications as manufacturing.

Terahertz technology has been promoted for an astonishingly wide range of applications:

• Manufacturing: real-time, in-situ process control, product inspection and material evaluation
• Food: food inspection for spoilage and contamination, determining the water content of food
• Biomedicine: mammography, bone tomography, endoscopy, medical diagnostics, detection of skin cancer and other diseases, identification of drugs or other substances in the blood, genetic sequencing
• Security and defense: detection of concealed weapons and explosives; evaluation of biological threats; airline passenger screening; detection of contraband in luggage, shipping containers; inspecting or reading unopened mail
• Imaging: imaging the contents of packages, sealed documents or closed books, fossils or oil encased in rock
• Scientific: environmental sensing, pollution detection, plasma diagnostics, chemistry and biochemistry
• And many more.

Terahertz radiation's main advantage is its ability to penetrate an extraordinary range of materials. It has been used to image through drywall to locate studs and wiring; to peer inside a closed bottle of tablets to ensure their quality without disturbing the contents; to measure the moisture content of packaged cigarettes; to image through plastic, paper, cardboard and most common fabrics.

The versatility of terahertz radiation has opened up important opportunities in inspecting and evaluating materials and products during and after manufacture, to ensure that quality standards and technical specifications are met. Inspection can be directed at finished (and often packaged) products or materials, or at an intermediate or final stage of manufacture. There are many materials that are not amenable to terahertz inspection, but the sheer number that can be inspected is enormous.

Pharmaceutical inspection (primarily for tablets) is one promising application for terahertz systems, largely because the application has already been commercialized.

For the billions of pharmaceuticals produced every year, quality is a critical concern since both effectiveness and safety can suffer from incorrect concentration or even distribution within a tablet. Inspection is particularly important in controlled or timed release tablets, a rapidly growing segment of the industry. Such drugs present a number of difficulties, both in design and manufacture, and well known medications have been pulled off the market after it was found that the tablets could easily split apart.

For timed release, coated and even homogeneous tablets, manufacturers need to ensure that the tablet contains not only proper dosage but it is evenly distributed, with coatings and other structures intact. Just as ingesting crushed tablets can be hazardous because active ingredients are absorbed into the bloodstream too quickly, the same could be true of tablets with defective coatings or other structural flaws. Inspection can also be used to establish the authenticity of a product, since counterfeit tablets often have inferior coatings.

Tablets are generally quasi-transparent in the terahertz range, making them perfect candidates for terahertz scanning, where other bands such as RF will either be completely reflected or absorbed, or pass through with no interaction. Terahertz wavelengths are also compatible with the resolution needed to resolve tablet coatings. Terahertz imaging can provide a three-dimensional chemical and structural map of tablets without destroying them, even after the tablets have been packaged.

Faulty processing can also be detected. For example, the terahertz absorption spectra of some common pharmaceuticals will change significantly after the sample has undergone heat treatment, where the infrared spectra remain virtually unchanged in the same circumstances.

The usefulness of terahertz technology in pharmaceuticals is representative of what it has to offer more broadly. Many potential real world applications, like determining in-situ the thickness and uniformity of insulation coatings on wire, detecting rust under layers of paint or finding cracks inside of plastic parts, are not glamorous and hence not widely publicized, but are fundamental needs in solid industries with high volume products.

Terahertz systems are under investigation for many applications in materials evaluation, including semiconductors, solar cells, composite materials, polymer films, dielectric films and others. Applications in semiconductor manufacturing are especially appealing, given the large potential market. Terahertz systems are well suited to evaluate both epitaxial wafers and the interconnects in packaged chips, where market volumes could be significant.

Inspection of finished products is certainly a potential high profile application for terahertz systems, but inspection of materials at an intermediate stage of product fabrication or, for some companies, as final products, may be at least as important as inspecting more complex finished products. Detecting defects like cracks or non-uniformities in materials, primarily an imaging task, is a natural job for terahertz systems.

Terahertz systems are suited for inspecting materials that have not finished processing. For example, demonstrations have been made using terahertz radiation to measure the thickness of wet paint. Voids in ceramics have been detected before the material was cooled off after thermal treatment, thus avoiding the need to wait for more than an hour for cooling before adjusting process parameters to prevent the voids, necessary for other methods like ultrasound imaging. Research has also yielded promising results in identifying solvents and other chemical impurities within ceramic matrices.

The combined market in process control, product inspection, material evaluation and related applications is probably the most promising of the emerging terahertz applications. The sector is almost sure to grow substantially, even in the worst case scenario of relatively slow technical progress. Terahertz technology can address very real and specific needs in manufacturing, and offers the sector capabilities that in many cases cannot be duplicated by competing technologies. Process control, product inspection and materials evaluation could potentially dwarf all other terahertz applications.

J. Scott Moore, Ph.D. is , President of Thintri, Inc. which provides business and market intelligence for a wide range of technologies .