A 2D Thz Si-MOSFET Image Sensor Based on In-Pixel Demodulation: Toward a Fully Integrated Low-Cost Real-Time Thz Imaging System
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A 2D THz Si-MOSFET image sensor based on in-pixel demodulation: toward a fully integrated low-cost real-time THz imaging system N. Monnier, J.-A. Nicolas, A. Boukhayma, J.-P. Rostaing, G. Sicard Univ. Grenoble Alpes, F-38000 Grenoble, France CEA, LETI, MINATEC Campus, F-38054 Grenoble, France Email: [email protected] M. Ney, F. Gallée Institut Mines-Télécom, TELECOM Bretagne, Lab-STICC (UMR 6285), 29238, Brest Cedex 3, France Summary: Since 2008, the smart integrated imagers laboratory (L3I) in CEA-Leti has focused on THz imaging using Si-MOSFET detection for low-cost integration. Several circuits were designed and tested and the last one achieves the following performances: a NEP of 533 pW and a responsivity of 300 kV/W at 270 GHz. Terahertz (THz) imaging provides the answer to many questions about detection and possibilities of studies in large fields of applications. The THz waves are non-ionizing and have highly useful proprieties for imaging. The CMOS technology is an alternative to hybrid THz image sensors like the bolometer-based ones, using a low-cost, standard process and working at room temperature. At the end of the 00’s, a first detector was design [1] according to the theory of the plasma wave detection developed at the end of the 90’s [2]. The couple bow-tie antenna and MOSFET detector (fig. 1.a) in addition with an in-pixel low noise amplifier (LNA), providing 31 dB gain, were implemented on chip to optimize different design sizes for a mono-pixel use. Finally, on this work, the best pixel yielded a responsivity of 5 kV/W (165 kV/W with the in-pixel LNA) and a noise equivalent power (NEP) below 10 pW⁄√Hz around 300 GHz and still 55 V/W and 900 pW⁄√Hz at 1.05 THz. It was used for raster-scanned images of objects visible on figure 1.b). a) b) Figure 1: a) Layout view of the couple antenna and detector without the LNA, b) A 300 GHz transmission image of different tree leaves obtained by raster-scanning. After [1] In 2012, taking benefit from the previous work, a 2D imager of 1k pixels [3] was designed in order to avoid the raster-scanning technique and to allow real-time acquisition process. The in-pixel read-out circuit (fig. 2.a) is composed of the former blocks and a variable gain amplifier (VGA), with a gain ranging from 0 dB to 40 dB, a highly selective filter (Q = 100) and a continuous time filter in the pixel. The amplitude of the THz waves is modulated at a frequency fmod with a 75 kHz to 225 kHz range and is then demodulated in the pixel according to the center frequency of the filters. This translation at fmod, higher than the noise corner frequency, reduces the flicker noise in the front-end and the high-Q filtering reduces drastically the thermal one. The 16-paths, high-Q, filter is controlled by an in-pixel digital sequencer, avoiding distributing switching signals through the entire array. Using dedicated test pixel, on which an input signal can replace the one from the detected THz waves and with a spectrum analyzer (Agilent N9020A) at the output, the influence of the 16-paths filter (which can be enable or bypassed) on the noise power spectral density (PSD) was measured as shown in fig.2.b). The chopper frequency, the same as the central frequency of the 16-paths filter, is chosen to be at 156 kHz. This high-Q filter reduces the noise PSD for more than 20 dB in the signal bandwidth compared to the response shaped by the Gm-C filter only. The RMS output noise, 푉푛,푅푀푆, is obtained by integrating this noise PSD over the bandwidth of the video amplifier [0 ; 500 kHz]. a) b) Figure 2: a) Bloc diagram of the in-pixel circuit for demodulation and formatting of the signal, b) Measured noise PSD at the output of the readout chain, in blue, when the 16-paths filter is by-passed, in green, when the 16-paths filter is activated and the noise floor in red. After [3] The responsivity and the NEP are the two main factors of merit (FoM) for THz image sensor characterization; their definition is given in the Table 1. Using the noise PSD with the 16-paths filter enabled and the sum of the voltages of the 31x31 pixels for both FoM, state of the art performances were achieved and are summarized in Table 1. Some images were obtained at 100 fps at 200 GHz (fig. 3.a) and at 2.5 THz (fig. 3.b). a) b) Figure 3: Experimental set-up and resulting images: a) (left) 100 fps sequence of a copper ruler (right) passing in front of a 200 GHz beam, b) (left) 2.5 THz image of a metallic ring held with a tape (right) These performances give promising outlooks for future applications and CEA-Leti continues working on performance improvements and additional feature implementations. Table 1: Summary of characteristics and comparison of the state of the art. After [3] This work Ref. [4] Ref. [5] 25 to 100 fps, 1 k 240 µm×240 µm 25 fps, 1 k 80 µm×80 µm Global 8×8, 83 µm×83 µm pixels pixels pixels Ring antenna and Baw tie antenna and MOSFET Patch antenna and MOSFET Detection device MOSFET Rviii=100 kV/W @200 GHz ∑ퟑퟏ∗ퟑퟏ 퐕 Rviii= 115 kV/W 퐢=ퟏ 퐨퐮퐭,퐢 300 kV/W @270 GHz Rviii= 4.5 kV/W @820 GHz 퐑퐯퐢 = @860 GHz 퐏퐛퐞퐚퐦 216 kV/W @600 GHz Input referred RMS noise : Input ref. RMS noise : Input ref. RMS noise : readout chain noise iii i 0.2 µV 2.45 µV 18.83 µV i 푽풏,푹푴푺 533 pW @270 GHz 12 nW 19.8 nW 푵푬푷 = 푹풗풊 732 pW @600 GHz @860 GHz @820 GHz i Calculated based on the mean NEP of 28 pW/Hz1/2 given in [5] over a bandwidth of 500 kHz, but only takes in consideration the thermal noise; ii Responsivity including on chip gain; iii Calculated based on the readout chain gain given in [4] References [1] Schuster, F. et al., "A broadband THz imager in a low-cost CMOS technology," Solid-State Circuits Conference Digest of Technical Papers (ISSCC), 2011 IEEE International , pp.42,43, 20-24 Feb. 2011. [2] Dyakonov, M. and Shur, M. "Detection, mixing, and frequency multiplication of terahertz radiation by two- dimensional electronic fluid, " Electron Devices, IEEE Transactions on, vol. 43, n°13, pp. 380-387, 1996. [3] Boukhayma, A. et al., "A 533pW NEP 31x31 pixel THz image sensor based on in-pixel demodulation," European Solid-State Circuit Conference (ESSCIRC), 2014, pp.303-306 Sep. 2014 [4] Al Hadi, R. et al., "A 1 k-Pixel Video Camera for 0.7–1.1 Terahertz Imaging Applications in 65-nm CMOS," Solid-State Circuits, IEEE Journal of, vol.47, no.12, pp.2999-3012, Dec. 2012 [5] Dae Yeon, K.; Shinwoong, P.; Ruonan Han; Kenneth, K.O., "820-GHz imaging array using diode-connected NMOS transistors in 130-nm CMOS," VLSI Circuits (VLSIC), 2013 Symposium on , pp.C12,C13, 12-14 June 2013 .