Terahertz scanner examines active semiconductors contactlessly

The change in charge carrier density in semiconductor materials influences the properties of electromagnetic radiation in the terahertz frequency range. A research group from the Australian Terahertz Engineering Lab is using this relationship to investigate p-n junctions of diodes and transistors during operation. They succeeded in doing this through the casing of the four diodes and transistors examined.

This is a first step towards a contactless and non-destructive measurement method that provides information about the function of chips. Such a method would be a security risk for some chips, for example for secure elements or smartcard ICs that process secret key material.

So far, however, the method achieves a relatively low resolution and the investigation takes too long to capture switching operations at higher clock frequencies.

Terahertz waves

The team around Bryce Chung from the Terahertz Engineering Laboratory at the University of Adelaide works with terahertz signals in the frequency range around 275 GHz, i.e. 0.275 THz. Although the frequency is only a fraction of 1 terahertz, they are referred to as terahertz waves.

The suitability of radiation in this frequency range for investigating semiconductor components has been known for many years. For example, another research team already investigated the doping profile of a transistor using terahertz near-field nanoscopy in 2008.

According to his publication on IEEE Xplore, the Australian team has now succeeded for the first time in imaging the p-n junctions of standard components during operation through their respective casings.

As test samples, however, the researchers chose discrete components that have been manufactured for decades and have huge internal structures compared to modern chips: diodes of types 1N4007 and 1N4148, the N-channel JFET 2N5485, and the NPN transistor BC548B.

Higher resolution, long scan duration

However, the researchers solved a significant problem: the signal at 275 GHz actually has too large a wavelength to image the tiny p-n junctions. Therefore, the experts used a special receiver technique to evaluate additional information from the reflected signal.

They scan the semiconductor under investigation in steps of 0.25 millimeters each. The complete scan of a square area with a side length of 1 centimeter took about 30 minutes.

Therefore, the experimental setup so far cannot analyze the functionality of complete chips with fine structures, millions of transistors, and higher clock frequencies.

(ciw)