Multi-frequency Terahertz (THz) detectors have shown great application potentials in THz imaging and sensing systems. For the first time to our knowledge, a novel dual-frequency THz detector with the stacked structure consisting of a silicon-based plasmonic antenna and a metal-based antenna in one compact unit is proposed and fabricated in standard CMOS technology. Compared with the metal antenna, the antenna based on heavy-doped poly-silicon materials enables the detector to excite localized surface plasmon resonance mode, making the effective absorption of the THz waves and thus resulting in the significant responsivity enhancement of the detector. The experimental results show a maximum voltage responsivity up to about 2000 V/W and 450 V/W, while the noise equivalence power is as low as 23 pW/Hz0.5 and 110 pW/Hz0.5 for the silicon antenna and metal antenna at the frequency of 220 GHz and 650 GHz, respectively. The presented dual-frequency detector can be easily implemented in a small size in favor of high-density array integration.
© 2019 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
CMOS Terahertz (THz) detectors have attracted great interest in room temperature THz imaging [1,2] due to their excellent advantages such as fast on-state response time, low noise equivalent power (NEP), low cost and high integration level. By the integration of MOSFETs or diodes with on-chip antennas as well as readout electronics [3–5], the THz signal can be detected. Although the one-band antennas in a frequency range from 0.1 THz to 1 THz had already been widely designed with the metal materials in the standard CMOS process, the wide-band detection is still hard to be achieved owing to the fact that the input impedance of the MOSFET detectors differs with the frequency band [6,7].
To achieve a better broadband property, several detectors operating at different frequencies are usually designed in one chip [8–11]. For instance, F. Simoens et al have designed THz imaging arrays with microbolometer [12–14]. By application of dual-antenna structures, the thermal detector presents ultra-wideband and enhanced detection. On the other hand, for the CMOS THz detector, an array with 11 different working frequencies ranging from 0.2 to 4.3 THz was designed in 150 nm CMOS technology . Nevertheless, the 2-D detector array with the antennas in different sizes occupies much more chip area, which inevitably increases cost and integration difficulty.
To overcome this shortcoming, it is desirable to realize a multi-frequency detector in one compact unit. Many researchers [15–18] have designed multi-frequency antennas utilizing multilayer metals in the back-end process [19–22]. However, their research work was only concentrated on the lower microwave frequency band. In this case, the on-chip antennas suffer from high radiation loss. Moreover, the electrical line connection between the antenna and detectors causes significant power loss during signal transmission, especially in the multi-band antenna structures. Fortunately, a new optical antenna structure instead of the traditional metal antenna can provide a feasible strategy to avoid the loss of power and radiation by converting the input light into the localized surface plasmon resonances. Semiconductor materials such as indium antimonide (InSb) [23,24] and silicon [25,26] have been reported to construct these resonant antennas at terahertz frequencies.
In this paper, an innovative dual-frequency antenna for CMOS THz detector is presented. A stacked structure consisting of a silicon-based plasmonic antenna and a metal-based antenna is proposed to form a dual-frequency compact unit. The fabrication of a dual-frequency detector is investigated and its performance is characterized. The experimental results indicate that, as compared to the conventional dual-frequency antenna, the response frequency regime and the span size between two response frequencies can be expanded significantly as well as a better gain and area utilization rate.
2. Design and simulation
2.1 Antenna structure
A novel dual-frequency THz detector, including a silicon antenna, a metal antenna, and a MOSFET mixer is proposed. The MOSFET mixer is allowed to work in a frequency band that is much higher than its cut-off frequency because of the plasma effect in the channel . The dimensionless parameter ω0τ governs the physics of the plasma wave, where the τ is the time of the electron momentum relaxation in the channel and ω0 is the fundamental plasma frequency. For Si MOSFETs at room temperature, ω0τ <<1, the device operates as a non-resonant detection, which is a broadband detection with a smooth response function of ω . The silicon antenna uses surface plasmon resonance effect to realize local electric field enhancement in the MOSFET channel region for the THz signal detection. Differently, the metal bow-tie antenna feeds the THz signal into the MOSFET mixer operating in a non-resonant mode through the metal wire.
Figure 1(a) illustrates the 3D stacked structure diagram of the integrated dual-frequency THz detector in 0.18μm standard CMOS technology. A 220 GHz silicon antenna with fan-rod structure (blue pattern) is realized with bottom polysilicon layer while a 650 GHz metal antenna with bow-tie structure is implemented using top metal layer (M6, yellow pattern). The two antennas are surrounded by SiO2 dielectric layer for isolation. A MOSFET mixer is placed in the center of the dual-frequency antenna. In addition, a ground plane with a metal-1 layer (M1, light blue pattern) is formed above the silicon antenna to prevent electromagnetic (EM) radiation from penetrating into Si substrate. In order to expose the silicon antenna to incident THz signal, a fan-shaped hole is drilled on the metal-1 ground plane. The cross-section of the dual-frequency THz detector is shown in Fig. 1(b). It is clearly seen that the gate of MOSFET is in the same layer with the silicon antenna. The metal bow-tie antenna is connected to the source and drain of the MOSFETs through via lines between metal layers 1-6. Note that the silicon plasmonic antenna is not directly connected to the MOSFETs, thus the impedance match between the metal bow-tie antenna and the MOSFETs will not be affected.
In particular, the silicon plasmonic antenna is modified to a special structure with both bow-tie and rod features, which enables high localized field enhancement for THz signal radiation. To ensure efficient energy coupling from the silicon antenna to the channel of the MOSFET mixer, the heavily doped polysilicon is selected as silicon antenna material, which is often used as the gate of MOSFET in standard CMOS process. Due to the large imaginary part of the relative permittivity together with its negative real part, the highly-doped polysilicon becomes a good material candidate for THz absorber applications [25,29]. According to Maxwell's equations, the propagation constant K at the infinite interface between semiconductor and dielectric can be calculated by:
According to Eq. (3), the plasma frequency is affected by free carrier concentration. The real part of permittivity of the high doping semiconductor could be negative at THz range. The negative values make it possible for the excitation of surface plasmon polaritons. For instance, when the doping level of polysilicon is as high as 1 × 1020/cm3, the estimated plasmonic wavelength λspp = 2π/K is about 324 μm for 0.32 THz signal (λ = 940 μm), which is much shorter than THz wavelength λ. Due to the fact that the wavelength of surface plasmon is smaller than that of an electromagnetic wave in free space, the size of the silicon plasmonic antenna can be significantly reduced compared with that of the metal bow-tie antenna at the same working frequency. Therefore, this silicon antenna structure is especially suitable for compact dual-frequency THz detector.
2.2 Simulation setup
The THz antenna acting as the key component of the THz detector plays an important role in receiving and transmitting THz signals. The better performance of the antenna, the stronger feed signal received by the MOSFET mixer, which leads to the higher responsivity of the detector. In order to obtain the optimal performance and minimize the size of the THz dual-frequency antenna, the antenna structures are designed and simulated by means of High-Frequency Structure Simulator (HFSS). Figure 2 shows the HFSS simulation model of the dual-frequency antenna. Here, the simulated antenna structure is established based on SMIC 0.18 μm standard CMOS technology. The total volume of the simulation element is set to 1.9 × 106 μm3. The dual-frequency antenna is located in the center of the simulation element. The silicon plasmonic antenna is made of heavily doped polysilicon material where the doping concentration and thickness are 5.5 × 1019 cm−3 and 200 nm, respectively. The relative permittivity is calculated based on the Drude model. The bow-tie antenna and it's ground plate are all made of metallic aluminum with a thickness of 2.17 μm and 0.53 μm. In addition, the doping concentration of the Si substrate is 1016 cm−3, and its relative dielectric constant is 11.9. Aside from the substrate and the antenna structure, the rest space in the simulation element is filled with SiO2 material whose relative dielectric constant is 4.
In the HFSS simulation, the upper and lower surface of the model space is chosen as the radiation boundary with two pairs of master-slave boundaries around it and the operating frequency is swept from 150 GHz to 750 GHz. The geometry parameters of the dual-frequency antenna are shown in Fig. 3. For the silicon plasmonic antenna, the important geometry parameters include half edge length (L), sector angle (θ), length and width of rod (d and W), and gap distance of rod (g), while the antenna length (W), the bottom length of a trapezoidal patch (Z), the length and width of a rectangular patch (S and a) are the key geometry parameters for metal bow-tie antenna. All the geometry parameters will be optimized by HFSS simulation.
2.3 Simulation results
In the silicon plasmonic antenna simulation, the simulation excitation source is located above the antenna and a TM mode plane wave is radiated with frequency in the range from 150 GHz to 350 GHz where the electric field direction is parallel to the y-axis as shown in Fig. 2 and the electric field strength E0 is set to 1V/m. Figure 4(a) shows the simulated electric field profile near the silicon antenna surface. It can be seen that the plasmon resonance of the antenna surface is effectively excited and the rod region near the antenna gap has a high local field enhancement due to the coupling effect. When the geometry parameters of SPR antenna are optimized as d = 52μm, g = 2μm, L = 80μm, and θ = 160°, the central local electric field gain Ex/E0 is up to 300 times at the 220 GHz resonant frequency. It is also found, the rod gap distance has a great influence on the central resonance frequency and the local electric field enhancement for the SPR resonance antenna. Figure 4(b) shows that the peak of E/E0 varies with central resonance frequency for different g values. It is clearly observed that the resonance peak gradually shifts to the low-frequency band and the local field is enhanced when the gap distance of g is decreased. As shown in the inset of Fig. 4(b), when g is 2 μm, a maximum electric field enhancement at the central resonance frequency is obtained. A dipole model can be used to analyze this phenomenon. Because the positive and negative charges are accumulated at both ends of the antenna under the excitation of the incident light source, the two rods of the antenna can be regarded as a dipole with a capacitor structure in the middle. The smaller the rod gap, the larger the capacitor, the more electrons can be stored, leading to the larger central electric field. Taking into account the limited size of the MOSFET transistor, the gap distance of 2 μm is finally adopted for device fabrication.
As for the metal bow-tie antenna, it must be carefully designed to match the input impedance of the MOSFET transistor. We first calculate the input impedance of the MOSFET at 650 GHz by device simulation with Cogenda VisualTCAD tool. In this TCAD simulation, the channel length and width of the MOSFET are 0.18 μm and 1 μm, respectively. The thickness of polysilicon gate and gate-oxide layer are 200 nm and 3.9 nm, respectively. Using such actual device parameters, the simulated real and imaginary parts of the input impedance are 124 Ω and −271 Ω respectively. After that, the bow-tie antenna that matched with the input impedance is optimized by aid of HFSS simulation. In the HFSS simulation, a lumped port excitation source is situated at the center of the metal bow-tie antenna, in which, the signal frequency is tuned in the range of 550 GHz to 750 GHz.
The HFSS simulation results reveal that when the antenna structure parameters of S = 142 μm, Z = 70 μm, W = 178.4 μm, and a = 4 μm are selected, the optimal gain of 6.3 dB and the low return loss S11 of about −36 dB are achieved at the 650 GHz resonant frequency, indicating good impedance matching between the bow-tie antenna and MOSFET. Since the metal bow-tie antenna applies the ground layer structure with a hole, different from the traditional ones, the incomplete ground layer will inevitably have an impact on the performance of metal bow-tie antenna. Figure 5(a) compares the return loss S11 of metal bow-tie antenna with and without a hole in the ground layer. It is found that for the ground layer without a hole, the metal bow-tie antenna exhibits return loss S11 of −29.3 dB at 650 GHz. When two sizes of fan-shaped holes are adopted, S11 is changed to −28.1 dB and −36.9 dB respectively, informing the additional hole in the ground plane almost doesn’t damage the S11 parameter of the metal bow-tie antenna. However, the radiation patterns are degraded by the additional hole in the ground layer, as shown in Figs. 5(b) and 5(c). When there is no hole in the ground layer, the front-to-back ratio and gain at 650 GHz are 22.63 dB and 3 dB, respectively. For the ground layer structure with a small hole with an area of 8762 μm2, the corresponding values are reduced to 3.71 dB and 1.8 dB. However, when a larger hole with an area of 17085μm2 is used, the gain is significantly improved to 6.3 dB and the front-to-back ratio is also enhanced to 4.31 dB. We, therefore, choose the 17085 μm2-sized fan-shaped hole based on considerations of both return loss and radiation patterns to realize better performance for the 650 GHz metal bow-tie antenna. The optimal structure parameters of the dual-frequency antenna are listed in Table 1.
3. Fabrication of terahertz detector
On the basis of the above simulation results, the fabrication of dual-frequency antenna coupled with NMOS transistor was implemented using 0.18μm standard CMOS process with six metal layers. The process flow is illustrated in Fig. 6. As shown in Fig. 6(a), after the formation of shallow trench isolation (STI) and deep P-well on silicon substrate, the silicon dioxide layer and the doped polysilicon layer are deposited on the top of STI successively. Then, by applying the reactive ion etching (RIE) technique, the gate of the MOSFET transistor, as well as the silicon plasmonic antenna, are formed. The etched parts are illustrated in Fig. 6(b). After that, the N-type ions implantation is carried out on the source, drain, and gate of MOSFET. Subsequently, the terminals of MOSFET are silicided for Ohmic contact, as shown in Fig. 6(c). Above the polysilicon layer, the metal layers 1-6 are deposited in sequence. The inter-layer dielectric (ILD) and inter-metal dielectric (IMD) layers are inserted between the active region and metal layers. To reduce the antenna loss, the top metal-6 layer is used to fabricate metal bow-tie antenna and the metal-1 layer is used as a ground plane. As shown in Fig. 6(d), through via arrays between the metal layers, the metal bow-tie antenna is connected to the source and drain of MOSFET. In the overall process flow, there is no additional process change in the standard CMOS technology. Figure 7 depicts the micrograph of the fabricated dual-frequency THz detector, which only occupies a small area of 0.187 × 0.142 mm2. It means the high-density array integration can be easily realized with low-cost standard CMOS process.
4. Experimental results and discuss
After the fabrication of the novel dual-frequency THz detector, the response voltages (ΔV) were experimentally characterized. The measurement has been performed in two separate steps from 190 GHz to 320 GHz and from 650 GHz to 690 GHz for silicon and metal bow-tie antennas, respectively. The THz sources with line polarization characteristic are generated by VDITx187 generator with a square-wave chopper. The induced voltage response of the detector was measured between the drain and source of MOSFET using a lock-in technique under zero drain current boundary condition. The RV is then calculated by the following formula :
Figures 8(a) and 8(b) show the measured response voltage ΔV of the THz detector as a function of gate voltage at two different frequency regions, respectively. It is seen that both in the low and high measured frequency regions, the voltage response ΔV increases firstly and then decreases with the increase of gate voltage, showing a maximum value near the threshold voltage VTH of the MOSFET transistor. This characteristic is in good agreement with the plasma wave detection theory for MOSFETs THz detectors [30,31]. According to Eq. (2), the frequency dependency of RV for two operating frequency regions can be obtained, as shown in the inset of Figs. 8(a) and 8(b). It is found that the high-frequency metal bow-tie antenna can respond to the THz wave in the frequency range of 620 GHz-690 GHz. The available bandwidth is about 50 GHz and the center response frequency is 650 GHz. For the low-frequency silicon plasmonic antenna, the measured center response frequency is 220 GHz with 40 GHz bandwidth. Apparently, the experimental results well accord with the simulation data for the dual-frequency antenna structure.
The experimental results indicate that the optimum operating frequencies of the dual-frequency detector are 220 GHz and 650 GHz for silicon and metal bow-tie antennas, respectively. Figure 9 further illustrates RV as a function of VG at the frequencies of 220 GHz and 650 GHz. The maximum responsivity of metal bow-tie antenna at 650 GHz is about 400 V/W. By comparison, the THz wave response gain of the silicon antenna is greatly increased and the maximum responsivity is up to 2000 V/W at 220 GHz.
As the polarization of the bow-tie metal antenna has been widely studied, here we focus on the polarization sensitivity of the silicon plasmonic antenna. The polarisation of the optical antenna is studied through changing the angle Φ between the incident beam and the optical antenna. As shown in Fig. 10, the voltage responsivity Rv exhibits a strong dependence on Φ, informing the optical antenna is a typical linear polarization pattern.
The noise voltage fluctuations spectral density SV was measured by Agilent 35670A signal analyzer, as shown in Fig. 11. SV is dominated by 1/f noise at low frequency. When the frequency is larger than 1 KHz, the 1/f noise is significantly reduced and the thermal noise and generation-recombination noise become major noise contributors. Meanwhile, it can be observed that the noise of the detector is reduced with the increase of VG. When the modulation frequency is 1 KHz, the noise of the detector is about 50 nV/Hz0.5 at the VG of 0.4 V. To suppress the low-frequency 1/f noise, the detector is tested under 1 kHz modulation. From the measured SV and RV data, we finally obtain the NEP value by applying 
Figure 12 shows the gate voltage dependence of NEP at the frequency of 220 GHz and 650 GHz. With the increase of gate voltage, NEP decreases significantly and then tends to stabilize gradually. At the operating gate voltage of 0.4 V, the NEP values at 220 GHz and 650 GHz are about 23 pW/Hz0.5 and 110 pW/Hz0.5, respectively.
Table 2 compares the antenna size, responsivity and NEP characteristics between the proposed dual-frequency THz detector and the current reported CMOS Terahertz detectors in CMOS technologies. The proposed new THz dual-frequency detector exhibits excellent performance in response sensitivity, noise equivalent power, and occupied area. It is particularly interesting to note that the 220 GHz silicon plasmonic antenna has significant advantages over the reported dual-frequency metal bow-tie antennas with higher gain, lower noise, and smaller size. The responsitivity performance is even better than that of the current reported single-frequency silicon plasmonic antenna. Furthermore, the proposed silicon plasmonic antenna does not need to match the MOSFETs input impedance, thus the perfect impedance match between the metal bow-tie antenna and the MOSFETs is easily realized, which greatly reduces the energy loss of antenna transmission and improve the performance of the 650 GHz metal bow-tie antenna. More importantly, the proposed stacked structure occupies less area than the reported dual-frequency antennas and even the single-frequency antennas, which enables high integration of the THz array detector.
A novel and compact dual-frequency CMOS THz detector including two kinds of antenna structures is firstly introduced in which the silicon plasmonic antenna and metal bow-tie antenna are designed using polysilicon and metal layers respectively in 0.18 µm standard CMOS process without additional process steps. These two dual-frequency antenna structures are optimized by HFSS simulation and the feasibility is verified. The experimental results show that at 220 GHz and 650 GHz operating frequencies, the responsivity of silicon antenna and metal bow-tie antenna is up to 2000 V/W and 450 V/W, respectively, achieving high response gain. When the modulation frequency of the THz wave is 1 kHz, the NEP is as low as 23 pW/Hz0.5 and 110 pW/Hz0.5 for silicon antenna and metal bow-tie antenna respectively. In particular, the length of the silicon antenna is only 162 µm, which is significantly smaller than the size of the reported multi-frequency antennas. Therefore, the presented compact design scheme provides a promising candidate for the realization of high-density and high-performance dual-frequency THz detectors in cost-effective CMOS technologies.
National Key R&D Program of China (2016YFA0202102 and 2016YFB-0402403); the open project of Key Laboratory of Infrared Imaging Materials and Detectors (IIMDKFJJ-17-07).
The authors declare that there are no conflicts of interest related to this article.
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