This paper investigates terahertz detectors fabricated in a low-cost 130 nm silicon CMOS technology. We show that the detectors consisting of a nMOS field effect transistor as rectifying element and an integrated bow-tie coupling antenna achieve a record responsivity above 5 kV/W and a noise equivalent power below 10 pW/Hz0.5 in the important atmospheric window around 300 GHz and at room temperature. We demonstrate furthermore that the same detectors are efficient for imaging in a very wide frequency range from ~0.27 THz up to 1.05 THz. These results pave the way towards high sensitivity focal plane arrays in silicon for terahertz imaging.
© 2011 OSA
Terahertz (THz) systems and technology have become of large interest over the last 10 years. THz rays are non-ionizing and present an alternative to X-rays for imaging through paper, cloths and many plastic materials. THz imaging applications suffer however from the lack of low-cost detector arrays fast enough for video-rate imaging.
The first demonstration of sub-THz and THz detection by CMOS field effect transistors (FETs) in silicon was made in by Knap et al. . Tauk et al. have shown later that these devices can reach a noise equivalent power (NEP) competitive with the best conventional room temperature THz detectors  while offering the advantages of room temperature operation, very fast response times, easy on-chip integration with read-out electronics and high reproducibility leading to straightforward array fabrication. One significant step towards improvement of the responsivity of these detectors concerns the coupling of the free-space THz wave to the transistor channel. Since the wavelength of the THz wave is much larger than the dimensions of the transistor, metal grating coupler [3–6] or integrated antenna structures are required to ensure an efficient coupling. The first focal plane arrays of CMOS THz detectors with integrated antennas and amplifiers have been designed and used for a few pixel imaging at ~600 GHz by the Pfeiffer and Roskos groups with a responsivity of up to 1.1 kV/W and a NEP of 50 pW/Hz0.5 [7–9].
In this work we present MOSFET detectors with integrated on-chip antennas that can reach a record responsivity of ~5 kV/W and a NEP below 10 pW/Hz0.5 in the atmospheric window around 300 GHz. We demonstrate also that the same detectors can be used for THz imaging in a very wide frequency range: from 270 GHz to above 1 THz.
2. Detection principle
The THz detection phenomenon in FETs was first explained by the Dyakonov-Shur plasma wave theory [10–12]. When THz radiation is coupled to the FET – between gate and source – the ac THz voltage modulates simultaneously the carrier density and the carrier drift velocity. As a result, the THz signal is rectified and leads to a dc photoresponse ΔU between source and drain proportional to the received power. For high carrier mobility devices (e.g. III-V devices at cryogenic temperatures) the THz field can induce plasma waves that propagate in the channel leading to a very efficient resonant narrowband detection. In silicon and at room temperature, plasma waves are usually overdamped leading to a non-resonant broadband detection. In this case the THz radiation only causes a carrier density perturbation that decays exponentially with the distance from the source and the dc photoresponse ΔU between source and drain can be written as:12]. The case of room temperature, non-resonant detection can be explained alternatively by the model of distributed resistive self-mixing [7,8]. Although not treating all plasma related physics rigorously, the resistive mixing model allows – for the non-resonant regime – a rational detector design and efficient detector simulation with standard FET circuit models [13,14].
3. Implementation and measurements
We have designed and processed the THz detectors in a standard industrial 130 nm CMOS technology on bulk silicon substrate with a resistivity of ρ = 10 Ωcm. Figure 1(a) shows a micrograph of the test chip. The basic detector consists of a broad band bow-tie antenna in the metal interconnect layers connected to a nMOSFET. We realized different detector designs by varying the FET dimensions, the antenna geometry, and the antenna connection (to gate and source or drain and source terminals). The width W and the length L of the detecting FETs have been varied within L = 130 – 300 nm and W = 0.25 – 15 µm. We reduced the substrate thickness from 370 µm to 125 µm and mounted the dies in gold plated ceramic chip carriers. Figure 1(b) shows schematic views of different detectors and their characteristics.
For the photoresponse measurements and imaging we used an electronic 292 GHz source based on frequency multipliers and three backward wave oscillator (BWO) sources covering the ranges from 270 to 500 GHz and from 0.85 to 1.09 THz. The experimental set-up is shown Fig. 2 and it is similar to the one used in Ref. . The radiation is collimated and focused by off-axis parabolic mirrors, the radiation intensity is mechanically chopped at 333 Hz and the photo-induced drain-source voltage ΔU is measured using a lock-in technique. A miniature LED in combination with an indium tin oxide (ITO) mirror helps with the alignment of the source. All measurements were done at room temperature.
To determine the responsivity R v of a pixel we have raster scanned the source beam and used the method of Ref. :2], the dominant noise source in non-current-biased FET detectors is thermal noise from the transistor channel. Therefore we have estimated the NEP of the detectors as:
4. Results and discussion
Figure 3 shows the responsivity and NEP for a few detectors and substrate thicknesses at 292 GHz. The best results were obtained with the detector C14 with the minimum FET dimensions – 130 nm gate length, 250 nm gate width – and the bow tie connected to source and gate. The minimum NEP reaches a record 8 pW/Hz0.5. This value is the best reported so far for FET THz detectors in silicon. It is better than those of commercial room temperature THz detectors such as Golay cells (200 – 400 pW/Hz0.5) or pyroelectric detectors (400 pW/Hz0.5). The inset of Fig. 3(b) presents a raster scan image of the source beam at 292 GHz and a gate bias of V gs = 0.2 V. One can see that the maximum signal is ~20 mV with a total beam power of 2 mW. The beam is Gaussian-like with a full width at half maximum (FWHM) of ~4.6 mm.
In Fig. 4 we have plotted the responsivity of C14 as a function of frequency at a gate bias of V gs = 0.2 V. There is a sharp responsivity maximum around 295 GHz, with a peak value of 5.3 kV/W. This is the highest reported responsivity for a FET detector without on-chip amplification. It is worth to stress however that very high responsivities above 1 kV/W were obtained also for other detectors on the same chip such as C5 (see Fig. 3(a) and Fig. 1(b)). The inset of Fig. 4 displays a raster scan image of the BWO beam at 1.05 THz. The responsivity still is 55 V/W and the NEP 900 pW/Hz0.5 even at that high frequency showing the applicability of the detector for imaging in the whole ~0.27 THz to ~1.05 THz frequency range.
In order to demonstrate the imaging capability of the detectors we took raster scanned transmission mode THz images of different objects. Figure 5 shows an image at 292 GHz of a wrapped chocolate bar with a metallic needle inside. The needle as well as the break lines of the chocolate appear clearly in the THz image with a reasonably good spatial resolution.
The record responsivity and NEP in this work were obtained thanks to the careful design of the FET and its integration with an antenna. The first important fact was the choice of the gate length (130 nm) close to a few times the characteristic length l c that defines the exponential build-up of the rectified dc voltage away from the source [11,12]. This way, the channel length is limited to the actively rectifying part avoiding useless series resistance of a non-active part. For too long transistor channels this series resistance generates significant additional noise degrading the NEP and it decreases the responsivity if the input resistance of the measurement equipment is not substantially higher (voltage divider/ loading). The second important fact was the asymmetric antenna connection to the gate and source terminals as shown in Fig. 1(b). In fact, any parasitic THz signal on the drain side leads to a compensating dc voltage (rectification at the source side) that decreases the responsivity dramatically. This can be clearly seen when comparing the responsivity of C14 and C15 in Fig. 3(a). The only difference between the detectors is that C14 has the antenna connected to gate and source and C15 to source and drain. In this case the responsivity is reduced by more than one order of magnitude.
The third important fact was the thinning of the silicon substrate from 370 µm to 125 µm which led to an increase of the responsivity of about one order of magnitude – see Fig. 3(a). This is probably due to the reduction of the losses in the substrate. The narrowness of the peak at 295GHz in Fig. 4 suggests also the possibility of standing wave phenomena. Our test chip is a silicon slab with 2 mm x 2 mm surface dimensions and its thinning to 125 µm can lead to the formation of a simple optical cavity. In this case the substrate slab works as an electric field condenser leading to a local increase of the electric field amplitude.
Although the performances reported in this work are record values, still a big progress can be achieved. Ways of improvement can be the use of a silicon on insulator technology with a high resistivity substrate or the use of resonant, high gain antennas. Also the formation of high quality factor silicon cavities can be considered.
In conclusion, we have shown that with a careful choice of the FET’s gate length, a reduction of substrate losses and a proper connection of the antenna, a record NEP (below 10 pW/Hz0.5) and responsivity (above 5 kV/W) are achievable for detectors operating in the atmospheric window at 300 GHz. We have also demonstrated the imaging capability of the same detector in a very broad frequency range from 0.27 to 1.05 THz. These results show that Si-MOSFETs are among the best candidates for room temperature focal plane arrays for THz detection and imaging.
The authors thank S. Rumyantsev for helpful discussions on noise measurements, P. Kopyt for helpful discussions on antenna design, and B. Dupont for circuit design support.
References and links
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