Cool sensor detects infrared

Dave Wilson reports on an infrared detector unveiled at the Hannover Fair that could change the way gas spectrometers are designed

Identifying gases and measuring their concentration in a mixture is important in a number of applications such as gas leak detection, process monitoring and control and pollution control. Many gases can be uniquely identified by their optical absorption spectra, wavelengths and the knowledge of the absorption strengths.

Up until now, however, purchasers of infra-red gas spectrometry equipment only had two choices: either they could purchase expensive high-resolution instruments or, at the other end of the scale, inexpensive low resolution systems.

Now, the advent of pyroelectric arrays presents an alternative solution that combines low-cost with high-resolution. The fact that no cooling of the pyroelectric detector is required when using these devices greatly reduces system costs. Better yet, the signal to noise ratio of the devices at room temperature still provides ppm resolutions in the 3-5 micro m and the 8-12 micro m wavelength range.

By using thin film technology, Bert Willing and Paul Muralt, working at the Ecole Polytechnique Federale de Lausanne (EPFL) in Lausanne have fabricated pyroelectric detectors as linear arrays ranging in size from 2 to 128 pixels. The pyroelectric arrays themselves use a thin (1 micro m) ceramics film suspended on a membrane and show a high absorption (>90%) in a wavelength range of 0.6-20 micro m.

The detector unit features a wire bonded, dedicated 16 channel CMOS readout circuit that produces even less noise than a pyroelectric detector. What is more, it can deliver digital output signals with a 12 bit resolution. Readout of the latest 1 x 64 pixel array that the team have developed takes a total of 1sec, with the signals from the individual channels integrated over five cycles. The hybrid packaging of the sensor and CMOS electronics means that the same electronics module can be used for any type of sensors.

Because linear arrays represent a very niche market where each customer requires a slightly different design, this method of design has certain advantages over other methods, most importantly that in each case, only a relatively simple mask has to be changed to modify the design.

Measurements performed with a 1 x 64 array detector unit mounted to a standard 1/8m monochromator have yielded a spectral resolution of 240nm (3 pixels) with a cross talk between adjacent pixels of less than 20%, giving a line width broadening of less than 5%.

Figure 1 shows the absorption spectra of different mixtures of CO2/CO with detection limits of 3ppm for CO2 and 40ppm for CO, respectively. The incident detector power on a single detector pixel was as low as 50nW and was resolved with a resolution of 0.2nW. The readout of a complete spectrum took 1sec.

The developers at the EPFL feel that the uses for the devices will go farther than that of simply gas spectrometry. They claim that opportunities for design wins also exist in other electronic systems ranging from complex domestic systems to micromotion detectors.

EPFL Tel: +41 21 693 4957