SiGe technology significantly reduces price for millimetre wave spectroscopy
The institute claims it devised a cost-effective way to manufacture the SiGe semiconductors – a transmitter and a receiver with integrated antenna, working in the frequency range from 238GHz to 252GHz. Since these devices are manufactured in standard silicon technology, the process is basically compatible with the ones established across the semiconductor industry. This translates into low production cost for eventual series production and establishes a technology basis for a gas sensor at a price level hitherto impossible.
The application potential is huge, the institute says in a release. In safety applications it can be used to detect toxic gases. Other application fields are chemical process control – for instance for plasma etching in the semiconductor industry. Such sensors could also be used in the healthcare segment: Analysing the respiratory gas of a patient could help to early detect lung diseases.
Millimetre absorption spectroscopy is an established laboratory technique, utilised in the molecular spectroscopy as well as in radio astronomy to exactly determine the concentration of molecules. The RF sources traditionally used in this field – schottky diodes with downstream frequency multipliers – however are very expensive and clumsy. Since a couple of years commercial sources are available based on multiplying the frequency of millimetre waves. These systems are more compact but their price remains relatively high.
Recently a US research team introduced a gas spectroscopy system for the frequency range from 210 to 270GHz that has been built with commercially available millimetre wave components. The costs for such a system are currently determined by the high manufacturing costs for the millimetre wave components. The challenge therefore was to develop a sensor system based on integrated circuits and an established chip technology such as SiGe or CMOS – which would contribute to reduce the manufacturing costs significantly.
The IHP researchers developed receiver and transmitter prototypes in SiGe technology that work in the frequency range between 238GHz and 252GHz. "This is a relative narrow frequency range", a researcher admitted, but "after all, this is just a demonstrator, better implementations will follow", he said.
The demonstrator utilises an optical bench which carries the transmitter and the receiver module. The effective antenna gain is amplified through a lens. For the gas spectroscopic measurements, a gas absorption cell the size of 0.6m had been positioned between transmitter and receiver. The receiver IF signal has been recorded as a function of the transmitter frequency, using commercially available laboratory measurement technology. The researchers used an integrated local oscillator for transmitter and receiver likewise, whose frequency has been stabilised by an external PLL circuit. The two PLL units use two reference frequencies with constant offset to achieve a constant intermediate frequency for the receiver through a complete sweep. Thus, it is possible to detect even small amplitude changes as a result of gas absorption.
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