A 239-281GHz Sub-THz Imager with 100MHz Resolution by CMOS Direct-conversion Receiver with On-chip Circular-polarized SIW Antenna Yang Shang, Hao Yu*, Chang Yang, Yuan Liang and Wei Meng Lim
I. I NTRODUCTION Terahertz (THz) radiation (0.1-10THz) fills in the gap between electronics and photonics with unique spectroscopic properties for various material discriminations [1]. A great deal of attention has been paid to THz imaging system with biomedical applications due to the moderate wavelength of THz signal that can leverage advantages of both millimeterwave (mm-wave) and optics: high spatial resolution, good penetration depth to human tissue and no harmful ionization. However, the current time-domain optics based THz imaging systems are bulky, expensive with low detection resolution due to the spectral specific property of tissue samples. Recently with the advanced scaling of CMOS technology, it has become feasible to realize integrated THz circuits [2], [3], [4], [5] with low-cost CMOS process towards portable and large-arrayed THz imaging system. However, it is challenging to design a sub-THz imager that can detect a sub-THz signal, which is usually attenuated during propagation with significant loss. Diode detection based receivers [2], [3] have a wide detection band but limited spectrum resolution, which is hardly used for spectroscopy analysis. Super-regenerative receivers [4], [5] have high spectrum resolution but only in narrow detection band at the resonance frequency. In order to have a CMOS THz imaging system with high sensitivity, wideband and high spectrum resolution, the direct-conversion receiver architecture is required. In addition, the high-gain on-chip antenna with sufficient bandwidth is
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Abstract—A 239-281GHz imager by direct-conversion receiver is demonstrated in 65nm CMOS process with high spectrum resolution and high sensitivity for sub-THz imaging. The subTHz imager consists of a circular-polarized substrate integrated waveguide (SIW) antenna, down-conversion mixer and power gain amplifier (PGA). The SIW antenna is compact in area of 0.17mm2 with -0.5dB gain and 32.1GHz bandwidth. The single-gate mixer can achieve 80GHz bandwidth centered at 260GHz with a conversion gain of -19dB. High resolution signal detection is achieved by a three-stage PGA with 150MHz bandwidth. The proposed sub-THz imager is fabricated with measurement results: -2dBi conversion gain over 42GHz bandwidth, -54.4dBm sensitivity with 100MHz detection resolution bandwidth, 6.6mW power consumption and 0.99mm2 chip area. Moreover, frequency-dependent sub-THz images are demonstrated as well. Index Terms—CMOS 65nm, sub-terahertz, substrate integrated waveguide, imaging, biomedical, high detection resolution.
Intensity
School of Electrical and Electronic Engineering, Nanyang Technological University, Singapore, 639798 Contact Email:
[email protected]
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Fig. 1: Sub-THz image system with direct-conversion receiver for high spectrum resolution detection and circular-polarized antenna for the tolerance of depolarization effect.
also required to compensate the signal loss. The previous onchip antenna works [2], [3], [4], [5] have either low gain or narrow bandwidth as well as ignored polarization issue. For example, given the THz source with a linearly polarized radiation, the longitudinal polarization may be turned into transverse direction after penetrating through the tissue [1]. If the antenna at receiver is also linearly polarized, the detection efficiency may be largely reduced due to the mismatch in the polarization directions. In this work, a broadband CMOS sub-THz imager with direct-conversion receiver is developed with a circularpolarized substrate integrated waveguide (SIW) antenna, down-conversion a mixer and a power gain amplifier (PGA). The SIW antenna is in an area of 0.17mm2 with -0.5dB gain and 32.1GHz bandwidth. The single-gate mixer can achieve 80GHz bandwidth with a conversion gain of -19dB. The threestage PGA achieves 150MHz bandwidth for the detection resolution. The proposed imager is fabricated in 65nm CMOS with measured results: -2dBi conversion gain over 42GHz bandwidth, and -54.4dBm sensitivity with 100MHz detection resolution bandwidth. II. S UB -TH Z D IRECT- CONVERSION R ECEIVER D ESIGN The design of sub-THz imager with CMOS directconversion receiver has to be conducted in a scenario without any low noise amplifiers (LNA) due to the limited fmax as illustrated in Fig. 1. Firstly, a high gain on-chip antenna is required to effectively increase the signal-to-noise (SNR) ratio
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for detection; secondly, the noise contributions from mixer and PGA are no longer negligible, so the noise figure (NF) of receiver has to be improved by increasing the conversion gain of mixer and minimizing the NF of PGA. In this section, the designs of on-chip antenna, mixer and PGA in a sub-THz CMOS direct-conversion receiver are introduced.
strip line with characteristic impedance of 40Ω, which is implemented by M8 and M1 layers for signal and ground, respectively. The antenna structure is verified by a full wave simulation in Ansoft HFSS. Fig. 3a shows the simulated radiation pattern at 270GHz. It has a broadside radiation pattern with 6.2dB directivity and a 21.4% radiation efficiency at 270GHz. As a result, a -0.5dB antenna gain is obtained as illustrated in the simulated 3D radiation pattern in Fig. 3b. Fig. 3c shows the simulated E-filed distribution of SIW cavity at 270GHz, where a circular-polarized field distribution is observed. The simulated 3dB-axial-ratio frequency range is 265.6-274.5GHz as observed from Fig. 4. Fig. 4 also shows the S11 of the proposed antenna, where a very wide -6dB S11 bandwidth of 32.1GHz is observed.
A. Circular Polarized SIW Antenna with Corner Slots
B. Sub-THz Down-conversion Mixer
The proposed on-chip SIW antenna is designed in 65nm CMOS process with 9 metal layers as shown in Fig. 2. A composite dielectric material with silicon dioxide (SiO2 ) and silicon nitride (Si3 N4 ) is enclosed in the cuboid cavity (410µm × 410µm × 9µm) formed by the top most aluminum layer (AL), bottom most copper layer (M1) and metal walls constructed by metal layers and via bars (M1-AL). The chip area required for SIW antenna is 0.17mm2 . Two 17µm wide rectangular slots with different lengths (325µm and 360µm) are crossed at the center of AL layer to create four resonance modes with perpendicular polarization directions at 270GHz. A rectangular slot (120µm × 30µm) are created at each corner to reduce the antenna size at the desired operating frequency. The bottom side (M1) is implemented in mesh type that tiny square slots (3.5µm) are placed with 12µm pitch to satisfy the metal density rules. The antenna input is fed by a micro-
Fig. 5 shows the schematic of the down-conversion mixer design. A single-gate mixer utilizing the nonlinearity of transistors is less frequency dependent with better conversion efficiency than the conventional transconductance based gilbert-cell mixer in sub-THz. In addition, compared to the subharmonic mixer working with 1/3-LO [6], the conversion gain is much higher if directly mixing the RF and LO signals in fundamental tones. A wilkinson combiner implemented by coplanar waveguide (CPW) is deployed to combine the RF signal from antenna and the LO signal, and isolate LO and RF ports. The combined LO and RF signal is connected to the input of common-source stage (M1) biased in the subthreshold region (0.4V VGS ) by a compact composite matching network implemented by CPW and metal-oxidemetal (MOM) capacitors. The following common-gate stage is applied to improve the conversion efficiency as well as the
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III. M EASUREMENT R ESULTS stability. One LC resonator is connected for the biasing of mixer as well as the rejection of unwanted harmonics at the mixer output. Fig. 6a shows the post-layout simulation results of Sparameters and conversion gain. Good input matching and LO-RF isolation are observed with S11, S22 and S12 smaller than -10dB in 220-300GHz. The mixer also has a wide 3dB bandwidth from 220GHz to 300GHz, The conversion gain of the proposed mixer is determined by the available LO power at the mixer input. When the LO power is 0dBm, the maximum conversion gain is -19dB, which is more than 10dB higher than that of the subharmonic mixer design in [6]. Note that 0dBm is a typical power level that can be generated on-chip [7]. However, the conversion gain could be largely reduced when the LO power is low. As shown in Fig. 6b, the conversion gain will drop to -37dB at 3GHz when LO power is reduced to -20dBm. C. Power Gain Amplifier Fig. 7 shows the schematic of the proposed PGA with three cascode stages. In each stage, both transistors are biased in the saturation region (0.6V VG for M3, M5 and M7, and VG for M4, M6 and M8 are connected to VDD). The resonator is implemented by a 410fF metal-insulator-metal (MIM) capacitor and a 3.5nH spiral inductor, which has a maximized Q of 15 at 3GHz. A common-source output buffer is used to drive a 50Ω output impedance for the purpose of measurement. From the post-layout simulation, the proposed PGA has a maximum gain of 33dB at center frequency of 3GHz and a 3dB bandwidth of 150MHz and a noise figure lower than 4dB.
The proposed CMOS sub-THz imager with directconversion receiver is fabricated in Global Foundries (GF) CMOS 65nm process. The chip micrograph is shown in Fig. 8 with an area of 0.99mm2 . The fabricated receiver chip is firstly measured alone followed by the integration in a wideband subTHz imaging system. A. Direct-conversion Receiver The receiver operates under 0.8V power supply with overall power consumption of 6.6mW. As shown in Fig. 9a, the receiver chip is firstly measured on probe station (CASCADE Microtech Elite-300). A LO-signal (VDI) of 220-330GHz is directly applied via a waveguide GSG probe with 50µm pitch, and a RF signal is emitted by a 20dBi gain horn antenna placed right above the chip under-test by 10cm distance. The output IF signal is connected to another low-frequency GSG probe with 100µm pitch. The receiver output power is measured by a spectrum analyzer (Agilent E4408b). As shown in Fig. 9b, the proposed receiver is measured with an operating bandwidth of 42GHz from 239GHz to 281GHz and a maximum conversion gain of -25dBi. Fig. 10a shows the measured resolution bandwidth of 100MHz. This is slightly lower than simulated bandwidth of PGA because of additional LC resonator in the downconversion mixer. The best sensitivity (S) is found to be 31.4dBm at 250GHz as shown in Fig. 9b, where S is calculated by P SDnoise · B/G, P SDnoise is the measured output noise power spectrum density from spectrum analyzer, B and G are the receiver resolution bandwidth and conversion gain, respectively. Due to the loss of waveguide and probe (∼15dB) at LO input, the maximum LO power allowed at the mixer input is
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This Work [2] [3] [5] 65nm 0.13µm 0.13µm 65nm 239-281 280 280 201 DirectDiodeDiodeRegenerconversion detection detection ative Circular Linear Linear Linear Polarization 42 7 700 1.5 System BW (GHz) 0.1 7 700 1.5 Resolution BW (GHz) -25/-2* 31 Gain (dB) -31.4/-54.4* -26.9 -59.6 Sensitivity (dBm) 6.6 2.5 0.1 18.2 Power (mW) 0.03 3.8 0.25 0.99 Chip Area (mm2 ) ∗calculated results when 0dBm LO power is applied to the mixer. Parameters CMOS Technology Frequency (GHz) Detection Method
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Fig. 10: (a) Gain measurement results when sweeping RF frequencies with fIF = 283GHz −fRF ; (b) receiver sensitivity at 250GHz verse receiver resolution bandwidth.
about -25dBm, which largely affects the receiver performance in terms of conversion gain and sensitivity. According to relation between conversion gain and LO power illustrated in Fig. 6b, the compensated receiver gain is -2dB when LO power is increased to 0dBm. Similarly, the receiver sensitivity in the 0dBm LO condition is improved to -54.4dBm as illustrated in Fig. 9b. As shown in Fig. 10b, a -104dBm sensitivity can be achieved at 250GHz when the resolution bandwidth is reduced to 1kHz by external filters. The receiver performance is summarized in Table I and compared to other recent state-of-the-art CMOS sub-THz image receivers. The proposed receiver has much smaller detection resolution bandwidth when compared to the other detection methods. Especially when comparing to the superregenerative based receiver designs with resonant type narrow band detection, the resolution bandwidth is further increased by 15 times, while the system bandwidth is improved by 30 times. Moreover, the sensitivity of proposed receiver is comparable to the designs in other receiving topologies [5] when a 0dBm LO power is applied. B. Sub-THz Imaging The configuration of sub-THz image system is shown in Fig. 11a with samples placed between the source antenna and the proposed receiver. The samples under test are hold by a X-Y moving stage controlled by the testing program. The established image system is applied to study Panadol pills and animal skin sample in dry and moisturized conditions at 240GHz and 280GHz, respectively. The results of two imaging cases are shown in Fig. 11b. In the first case with Panadol pills, one can clearly differentiate between a moisturized and dry pills due to the strong water absorption at sub-THz frequencies. In the second case with animal skin samples, the moisturized area can be clearly identified from the surrounding dry area. Moreover, different images are obtained for the same substance at 240GHz and 280GHz due to the frequency dependent absorption ratio. IV. C ONCLUSION A CMOS sub-THz imager is demonstrated in this paper. The wideband, high sensitivity and high resolution sub-THz signal detection can be achieved by the CMOS direct-conversion receiver with integrated compact substrate integrated waveguide
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(SIW) antenna with circular polarization. The demonstrated sub-THz image system is able to capture images in 239281GHz with a resolution bandwidth of 100MHz, which shows great potential to build a portable and large-arrayed THz imaging system. R EFERENCES [1] M. Tonouchi, “Cutting-edge Terahertz Technology,” Nature Photonics, vol. 1, p. 97105, Febrary 2007. [2] R. Han, Y. Zhang, Y. Kim, D. Y. Kim, H. Shichijo, E. Afshari, and O. Kenneth, “280GHz and 860GHz Image Sensors Using Schottky-barrier Diodes in 0.13 µm Digital CMOS,” in IEEE International Solid-State Circuits Conference, Feb. 2012, pp. 254–256. [3] F. Schuster, H. Videlier, A. Dupret, D. Coquillat, M. Sakowicz, J. Rostaing, M. Tchagaspanian, B. Giffard, and W. Knap, “A Broadband THz Imager in a Low-cost CMOS Technology,” in IEEE International SolidState Circuits Conference, 2011, pp. 42–43. [4] A. Tang and M.-C. Chang, “183GHz 13.5mW/pixel CMOS Regenerative Receiver for mm-Wave Imaging Applications,” in IEEE International Solid-State Circuits Conference, Feb. 2011, pp. 296–298. [5] A. Tang, Q. Gu, Z. Xu, G. Virbila, and M.-C. F. Chang, “A Max 349 GHz 18.2mw/pixel CMOS Inter-modulated Regenerative Receiver for Tri-color mm-Wave Imaging,” in IEEE MTT-S International Microwave Symposium, 2012, pp. 1–3. [6] S. Hu, Y.-Z. Xiong, B. Zhang, L. Wang, T.-G. Lim, M. Je, and M. Madihian, “A SiGe BiCMOS Transmitter/Receiver Chipset With On-Chip SIW Antennas for Terahertz Applications,” IEEE Journal of Solid-State Circuits, vol. 47, no. 11, pp. 2654–2664, 2012. [7] Y. Tousi, O. Momeni, and E. Afshari, “A 283-to-296GHz VCO with 0.76mW peak output power in 65nm CMOS,” in IEEE International Solid-State Circuits Conference, Feb. 2012, pp. 258–260.