New wave: Terahertz radiation will soon open new doors for manufacturers

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. Of the many potential applications of terahertz radiation, manufacturing is potentially the most promising.    

The definition of the terahertz portion of the electromagnetic spectrum has varied but is generally considered to be the band between infrared and microwave radiation, usually running from 300 GHz to perhaps 10 THz, overlapping those bands commonly referred to as the submillimetre and far infrared.

Terahertz radiation has long been an important concern in astronomy, given that approximately one half of the total luminosity of the universe and 98 percent of the photons emitted in the history of the universe lie in the terahertz portion of the spectrum. In addition, terahertz waves are not readily scattered by gas clouds in space, facilitating imaging at these wavelengths. 

Terahertz radiation offers capabilities generally unavailable in other bands. For example, terahertz technology offers the ability to image through a tremendous variety of materials. While the waves are reflected by metallic surfaces and absorbed by water, both of which remain opaque to terahertz signals, most other materials are transparent to some degree to terahertz radiation in at least some portion of the band. Terahertz systems can provide both images and spectroscopic data (possibly in the same measurement), as well as ranging data that can measure structures like coating or layer thicknesses, even in structures of many layers. The systems are generally non-contact, and so measurements can be performed on materials in-process, such as wet paint, or layered structures like roofing material.   

The capabilities offered by terahertz radiation have long been well known; the problem has been in their exploitation. The strengths of terahertz radiation, such as the ability to penetrate so many materials, has also made their generation and detection difficult and costly, relegating terahertz technology to specialty research applications, where there are no other options.  

However, a number of technical breakthroughs in photonics, electronics and nanotechnology achieved since the early 1990s have brought terahertz technology within striking distance of significant commercial markets like security, communications, manufacturing, medicine and electronics. Where bulk and ease-of-use have been longstanding issues, 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. Most importantly, costs continue to decline, making terahertz technology increasingly economically competitive with conventional methods in a number of applications.

While development must continue on systems and components such as sources, detectors, waveguides, and lenses, 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 commercial markets for terahertz systems in such promising areas as manufacturing.

Most applications take advantage of terahertz radiation’s ability to penetrate an extraordinary range of materials. It has been used to image through drywall to locate studs and wiring; to measure the moisture content of packaged cigarettes; to image through plastic, paper, cardboard and most common fabrics. 
Another interesting aspect of terahertz radiation is in its interaction with matter, which takes place generally via the motion of groups of relatively large molecules, like those encountered in biology. This opens up the possibility of detecting the signatures of an enormous number of specific chemicals as well as investigating biological processes.

One thing that makes this part of the spectrum so fascinating to many is that it is the range where nanoscale machinery has resonant frequencies. The most significant form of nanoscale machinery is, of course, biological molecules like proteins or DNA. Thus, there is the possibility of using spectroscopy in this range to distinguish one kind of DNA from another, or one protein from another.

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 conducted on finished (and often packaged) products or materials, or at an intermediate stage of manufacture. Of course 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, primarily because the application has already been commercialized by TeraView, which has sold commercial systems into that sector.  

For the billions of pharmaceuticals produced every year, quality is a critical concern since not only their effectiveness must be established but their safety as well, and both can suffer from incorrect concentration or even distribution within a tablet.

In manufacturing tablets, pharmaceutical manufacturers need to ensure that the active ingredient is the right amount and that it is evenly distributed, and that coatings and other structures are intact. With recent warnings that ingesting crushed tablets can be hazardous because active ingredients will be absorbed into the bloodstream too quickly, the same threat could be found in tablets whose coatings are not intact or uniform, or have other structural flaws.

Terahertz imaging can provide a three-dimensional chemical and structural map of a tablet without destroying it, even after the tablets have been packaged, and provide information on integrity of structures, uniformity of ingredients, etc. Faulty processing can also be detected, whether or not structures are intact. For example, the terahertz absorption spectra of some common pharmaceuticals will change significantly after the sample has undergone heat treatment, where the far infrared spectra remain virtually unchanged in the same circumstances. Inspection can also be used to establish the authenticity of a product, since counterfeit tablets often have inferior coatings.

Inspection of finished products is an obvious potential high profile application for terahertz systems, but inspection of materials at an intermediate stage of product fabrication may be at least as important. Detecting defects like cracks or non-uniformities in materials is a natural fit for terahertz systems and allows real-time correction of manufacturing processes.     

One of the most potentially profitable applications for terahertz systems lies in materials evaluation, where the technology is under investigation for many applications such as semiconductors, solar cells, composite materials, polymer films and dielectric films.

The non-contact nature of terahertz inspection is a big advantage in inspecting materials that have not finished processing. For example, demonstrations have been made using terahertz radiation to measure the thickness of wet paint. In another demonstration, voids in ceramics were detected before the material was cooled off after thermal treatment. This avoided the need to wait for more than an hour for cooling before process parameters could be adjusted to prevent the voids, as would be necessary for the conventional method, ultrasound imaging. The same benefits are likely in plastics manufacture.       

Applications in semiconductor manufacturing are especially appealing, given the large potential market. Terahertz spectroscopy has already been demonstrated to yield semiconductor wafer parameters including mobility, conductivity, carrier density and the presence of plasma oscillations.

Fault analysis remains a critical task in the manufacturing of advanced semiconductor circuits. These faults can occur in both the substrate wafer and the circuitry. Terahertz systems have been demonstrated to reveal defects in these materials, and could find significant markets in that sector. The viability of terahertz semiconductor wafer and circuit inspection has been a controversial topic but appears to have largely proven itself, at least for interconnect inspection, where similar (millimetre wave) technologies are well established and the transition to terahertz systems would be smooth.

In terms of potential market volume, the manufacturing market in process control, product inspection, material evaluation and related applications is probably the most promising of the emerging terahertz applications. Even in the worst case scenario of relatively slow technical progress, markets can be expected to grow significantly. 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.
  

J. Scott Moore, Ph.D., is president of Thintri, Inc., a full-service consulting firm based in New York. You can reach him at smoore@cloud9.net.