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Abstract
In flip chip packaging, the performance of terahertz (THz) array detectors is directly influenced by the flip chip. In addition, predicting this effect is difficult because the readout circuits in the flip chip are very complex. In this study, to reduce the influence of the flip chip, we design a new type of double-slot antennas for THz array detectors. For comparison, we designed and analyzed dipole antennas with the same period. Numerical simulations showed that the coupling efficiency of the double-slot array antennas at approximately 0.6255 THz does not degrade, if the flip chip structure is changed. However, in the case of dipole array antennas with the same period of 250 µm, coupling efficiency was severely affected by the flip chip structure. These results revealed that double-slot antennas are more applicable to THz array detectors compared with dipole antennas, as they can more effectively reduce the influence of the flip chip. Furthermore, we integrated the double-slot antennas into Nb5N6 THz array detectors using the micro-fabrication technology. Measurement results indicated that double-slot antennas possess the advantages of facile preparation and large-scale integration, which provide great potential for THz array detectors in flip chip packaging.
© 2020 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
Terahertz (THz) detectors have recently become one of the key components in THz science and technology, which have gained great attention and been widely used in many fields, such as imaging, security screening, and spectroscopy [1–4]. To improve the sensitivity and response speed of a THz detector, the size of the detector sensor is usually designed with micrometer (or nanometer) dimensions; however, the THz signals received by the detector are small as well. Planar antennas, such as bow-tie antenna and spiral antenna, which can efficiently receive electromagnetic (EM) waves, have been widely studied and integrated with THz detectors to effectively couple the THz signals to the place of the detector sensor [5–8]. Compared with a single THz detector, THz array detectors possess the advantages of high efficiency and high speed in imaging and detection, making them a research hotspot [9,10]. To realize a wide application of THz array detectors, packaging of the THz array detectors is the key, which remains an urgent problem to be addressed.
Flip chip packaging is the most important method in the packaging technology of array detectors, because of its advantages of high density and multi-function integration [11–13]. However, the electrodes integrated with the array antennas, which are used to connect with readout circuits, severely affect the performance of the antennas in a large array structure [9], making the design process of array detectors more difficult and complicated. Furthermore, in the THz range, three challenges are faced when packaging array detectors using the flip chip technology. First, the wavelengths of the THz signals (30–3000 µm) are equal to the size of the flip chip, which inevitably causes interference [14–18]. In actual flip chip packaging, array detectors and readout circuits are usually connected via indium bump bonds, forming an air cavity (height is usually higher than 10 µm) [19,20]. Such a complex structure can further enhance the interference effect, thereby adversely affecting the response signals of the array detectors because of improper design. Therefore, designing the methods to reduce the interference effect on array detectors is an important problem in flip chip packaging. Second, the ground plane on the flip chip and the electrodes of array detectors could be staggered because of technical errors in flip chip packaging. Therefore, the bumps fail to connect the electrodes of array detectors with the ground plane [21], resulting in low yield of packaged array detectors. Third, numerous metal lines are connected with circuits in the flip chip to read the response signals of array detectors, which significantly affects the response signals of the array detectors. Predicting these effects is difficult because the arrangement of metal lines in the flip chip is complex [22].
To address these problems, we designed a double-slot antenna for THz array detectors in flip chip packaging. The double-slot antenna possesses a large metal area, ensuring that the metal bumps connect the electrodes of array detectors with the ground plane (or metal pad) on the flip chip, despite technical errors in flip chip packaging. It can effectively improve the yield of packaged array detectors without requiring designing of additional electrodes for antennas, thereby simplifying the design process. We simulated and analyzed the coupling efficiency of double-slot antennas deigned for THz array detectors, and compared it with that of the dipole antennas. Numerical simulations indicated that the double-slot antennas are more applicable to THz array detectors compared with dipole antennas, and can more effectively reduce the influence of the flip chip. Furthermore, we integrated the double-slot antennas with Nb5N6 THz array detectors using the micro-fabrication technology. The measurement results showed that the double-slot antennas possess the advantages of facile preparation and large-scale integration, indicating the great potential of THz array detectors in flip chip packaging.
2. Antenna design for THz array detectors
2.1 Design of double-slot array antennas
We designed the double-slot antennas for THz array detectors on a high-resistivity Si substrate; a unit cell is shown in Fig. 1(a). The double-slot antenna was divided into two large metal areas, which act as electrodes; this procedure not only simplified the design process without requiring designing of extra electrodes for the antenna, but also made the array detectors convenient to be connected with the flip chip. The THz detector (or film) could be designed in the center of the antenna, as indicated by the purple area. The length (W4) of the slot is usually equivalent to the half wavelength λ of the signal, i.e., W4=λ/2 and W4≫W5, where W5 is the slot width [23]. Moreover, the model was simulated using CST microwave studio, and analyzed in the frequency range 0.6–0.7 THz. The bottom and top boundary conditions were set as open (add space), and the others were set as periodic. A polarized plane wave (along the x-axis) with the electric field (E) amplitude of 1 V/m, propagating along the z-axis, was employed as the incident THz signal; this resembles the condition in which the signals are incident from the rear of the chip substrate. To ensure a resonant frequency of 0.6–0.7 THz, i.e., signals can be enhanced at the place of double-slot antennas, the substrate thickness H is an integer multiple of λ/2. Meanwhile, considering low cost and easy fabrication, we selected the substrate thickness as H=350 µm; for the double-slot array antennas with high resolution, the period P was designed in a small size, and because the antenna size was greater than λ/2, in the array structure, we considered the circumstance of the period P=250 µm. Under the optimized parameters of W1=170 µm, W2=200 µm, W3=40 µm, W4=70 µm, W5=5 µm, W6=3 µm, W7=5 µm, W8=12 µm, W9=20 µm, W10=10 µm, and W11=10 µm, the results of E at the center of the antenna (x=125 µm, y=125 µm, and z=350 µm) are shown in Fig. 1(b) (red line). The resonant frequency points appeared at 0.6255 THz, 0.6773 THz, and 0.6915 THz, and the corresponding E (f=0.6255 THz) distribution on the cross-section of the model was monitored, as shown in Fig. 1(c). The THz signals were effectively coupled to the center of the double-slot antenna. In actual flip chip packaging, the electrode size could change because of technical errors. Thus, we investigated the influence of W1 on the performance of double-slot array antennas. When we changed W1 from 160 µm to 180 µm, the resonant frequency points remained almost unchanged, and the corresponding peak values exhibited only minor variations. This indicates that the electrode size has only a small influence on the coupling efficiency of double-slot array antennas.
Fig. 1. (a) Unit cell of the double-slot antennas designed for THz array detectors. (b) Results of E at the center (x=125 µm, y=125 µm, z=350 µm) of the antenna for different values of W1. (c) Distribution of E on the cross-section of the model in a periodic unit cell at 0.6255 THz.
2.2 Design of dipole array antennas
A unit cell of the dipole antennas designed for THz array detectors is shown in Fig. 2(a) [24]. Based on the Babinet’s principle, the polarization directions of the dipole antenna and the double-slot antenna were perpendicular, i.e., the polarization direction was along the y-axis. In the design process, integrating electrodes and antennas was complicated and difficult because the electrode size was comparable to those of the antenna and the period, which severely affected the performance of the antennas. Two h-shaped structures were designed for the dipole antenna, as shown in the magnified image. The length L8 of approximately λ/4 was equal to L2, resulting in a high impedance state at the connection between the electrode and the metal line (length: L2). It could reduce the signal intensity in the electrode areas, and the signals at the center of the dipole antenna were effectively enhanced. Further, the model was simulated and analyzed using CST Microwave Studio. The length (L4) of the dipole is usually equivalent to the half wavelength λ of the signal, i.e., L4=λ/2 and L4≫L7, where L7 is the dipole width. The simulation conditions were the same as those for the double-slot antenna, except for the polarization of THz waves along the y-axis; the substrate thickness H and period P were maintained the same as well. Considering the flip chip packaging process, the electrode size is normally higher than 20 µm; here, we preliminarily chose the electrode size as L=50 µm. Under the optimized parameters of L1=30 µm, L2=25 µm, L3=12 µm, L4=65 µm, L5=13 µm, L6=10 µm, L7=4 µm, L8=23 µm, and L9=10 µm, the results of E at the center of the antenna are shown in Fig. 2(b) (blue line). The resonant frequency points occurred at 0.631 THz and 0.6877 THz. We monitored the corresponding E (f=0.631 THz) distribution on the cross-section of the model, as shown in Fig. 2(c). The THz signals were coupled mainly to the center of the dipole antenna. However, parts of the signal were dispersed in the substrate, indicating the interference effect of array antennas. Similarly, the electrode size could change based on different requirements or by technical errors in actual flip chip packaging. Therefore, we studied the coupling efficiency of the dipole antennas with different electrode sizes. When we changed L from 40 µm to 60 µm, the results of E at the center of the antenna obviously changed (Fig. 2(b)). The value at 0.631 THz decreased from 60 V/m to 30 V/m. Because array antennas had influence on the distribution of THz signals in the substrate, and the electrode size was comparable to the antenna, the performance of the dipole array antennas was severely affected by the electrode size.
Fig. 2. (a) Unit cell of the dipole antennas designed for THz array detectors. (b) Results of E at the center (x=125 µm, y=125 µm, z=350 µm) of the antenna for different electrode sizes. (c) Distribution of E on the cross-section of the model in a periodic unit cell at 0.631 THz.
3. Analysis of array antennas in flip chip packaging
In the structure of flip chip packaging, the double-slot array antennas and dipole array antennas were simulated and analyzed. For the double-slot array antennas, a periodic unit cell is shown in Fig. 3(a). The double-slot antenna on the substrate must face the flip chip (with thickness H1). On the surface of the flip chip (Fig. 3(c)), the metal pads with electrodes of same size were connected with the antenna by metal bumps, forming an air cavity of height h. In actual fabrication, Si wafers are usually chosen as flip chips because of their advantages of low cost and matured process flow. To reduce the model size in simulation and the effect of the flip chip on array antennas performances (namely decreasing the reflectivity of the flip chip), we chose H1=200 µm. In flip chip packaging, the height of the air cavity is generally larger than 10 µm; thus, we selected h=15 µm in the simulation. The results of E at the center of the antenna are shown in Fig. 4(a). Four resonant frequency points of approximately 0.6756 THz, 0.6505 THz, 0.625 THz, and 0.609 THz, appeared, as indicated by the green line (Si). It overlapped with the resonant frequency points at approximately 0.6255 THz and 0.6775 THz at the condition without flip chip, as indicated by the black line (Non). Its coupling efficiency at approximately 0.6255 THz remained almost unchanged, when double-slot array antennas were designed in the flip chip structure. Additionally, to read the response signals of the array detectors, metal lines were designed in the flip chip, as shown in Fig. 3(d). Here, we simply added five metal strips (width: g1) in the flip chip to imitate the metal lines; the interval between two adjacent metal strips was g2. The results of E, when g1=10 µm and g2=20 µm, are shown in Fig. 4(c). Similarly, as indicated by the purple line (line), two resonant frequency points (approximately 0.6774 THz and 0.6255 THz) overlapped with the results of those under the condition without flip chip. However, in actual flip chip packaging, the metal lines in the flip chip are more complicated; therefore, predicting the influence of metal lines on the property of array antennas is difficult. Therefore, to reduce this influence, the flip chip surface was almost totally covered with metal areas, as shown in Fig. 3(e); the gap between these two metal areas was g. The results of E, when g=5 µm, are shown in Fig. 4(e). It exhibits three resonant frequency points: 0.6265 THz, 0.6373 THz, and 0.649 THz (represented by blue line (Au)). The intensity of E remained constant at approximately 60 V/m, which is similar to the result obtained for 0.6255 THz under the condition without flip chip. This result clearly shows that double-slot array antennas could function effectively at approximately 0.6255 THz without degradation of performance under the conditions of the above-mentioned three different flip chip structures. Consequently, double-slot antennas are applicable to THz array detectors in flip chip packaging.
Fig. 3. Unit cell of (a) double-slot array antennas and (b) dipole array antennas in flip chip packaging. (c) Two metal pads designed on the flip chip, (d) metal strips added in the flip chip, and (e) flip chip surface almost covered by metal.
Fig. 4. For the double-slot array antennas in flip chip packaging, the results of E for the condition of (a) two metal pads designed on the flip chip, (c) metal strips added in the flip chip, and (e) flip chip surface almost totally covered by metal areas. For the dipole array antennas in flip chip packaging, the results of E for the condition of (b) two metal pads designed on the flip chip, (d) metal strips added in the flip chip, and (f) flip chip surface almost totally covered by metal areas.
For the dipole array antennas in flip chip packaging, a periodic unit cell is shown in Fig. 3(b). When two metal pads were designed on the flip chip surface (Fig. 3(c)), the results of E at the center of the antenna are shown in Fig. 4(b) (represented by olive line (Si)). Further, four resonant frequency points occurred: 0.646 THz, 0.673 THz, 0.683 THz, and 0.692 THz. However, none of these resonant frequency points overlapped with the resonant frequency points (0.631 THz and 0.6877 THz) under the condition without flip chip (indicated by black line (Non)). For the condition of adding metal lines in the flip chip (Fig. 3(d)), the results of E at the center of the antenna are shown in Fig. 4(d) (represented by red line (Line)). The signals at approximately 0.631 THz apparently decreased. The intensity of E at 0.6477 THz was 33 V/m, which is almost half of that (at 0.631 THz) at the condition without flip chip. For the condition of the flip chip surface almost totally covered by large metal areas (Fig. 3(e)), the results of E at the center of the antenna are shown in Fig. 4(f) (represented by brown line (Au)). The resonant frequency points appeared at approximately 0.651 THz and 0.683 THz, which significantly differ from those under the condition without flip chip. This indicates that the coupling efficiency of the dipole array antennas is severely affected by the flip chip structure.
4. Experimental results of Nb5N6 THz array detectors integrated with double-slot antennas
Using the micro-fabrication technology, we integrated the double-slot antennas with the Nb5N6 THz array detectors, as shown in Fig. 5(a). First, a 120 nm thick Nb5N6 film was deposited on the high-resistivity silicon substrate (ρ>1000 Ω · cm) covered with a 200 nm thick thermal oxidation SiO2 layer [16]. Then, the double-slot antennas were fabricated using 200 nm thick gold via photolithography and magnetron sputtering. Subsequently, the Nb5N6 film was patterned into a 3 µm (width) × 12 µm (length) rectangle after performing photolithography and reactive ion etching (RIE). The resistance of the Nb5N6 THz array detectors was approximately 1 kΩ. Finally, the RIE equipment was used to fabricate the air-bridge, which could efficiently reduce the thermal conductivity of the Nb5N6 THz array detectors.
Fig. 5. (a) Optical microscopy image of Nb5N6 THz array detectors integrated with double-slot antennas. (b) Black line indicates the results of E in the Nb5N6 film, and red dotted-line represents the measured Rv of Nb5N6 THz array detectors.
Further, we measured the voltage response of the Nb5N6 THz array detectors using two off-axis parabolic mirrors [25]. The frequency of the signal source, which was modulated by a 1 KHz TTL signal, could be tuned from 0.613 THz to 0.685 THz. An output power of approximately 0.5 mW was measured by a thermal power sensor (OPHIR, 3 A-P-THz). The response voltage of the Nb5N6 detector, which was biased at 0.4 mA, could be read using a lock-in amplifier.
The voltage responsivity (Rv) is related to the response voltage Δu by the equation Rv=Δu/Pd, where Pd is the signal power within the periodic unit area of the Nb5N6 THz array detectors. The measured results are shown in Fig. 5(b) (indicated by the red dotted line). The maximum Rv of 312 V/W occurred at 0.64 THz with a full width at half maximum (FWHM) of about 10 GHz. Due to the thickness error of the Si substrate, the measured results differed slightly from the simulated results (indicated by black line). There were two resonant frequency points: 0.6255 THz (E: 59 V/m) and 0.677 THz (E: 71 V/m), and the corresponding FWHM was 9 GHz and 1 GHz. The resonant frequency point shifted at 14.5 GHz (from 0.6255 THz to 0.64 THz). Nevertheless, these results demonstrate that the double-slot array antennas, which possess the advantages of facile fabrication and large-scale integration, have a great potential in THz array detectors.
5. Summary
In this study, we designed double-slot antennas for THz array detectors applied in flip chip package. Meanwhile, for comparison, dipole antennas with the same period were designed and analyzed. Numerical simulations revealed that the performance of the double-slot array antennas at approximately 0.6255 THz does not degrade when the flip chip structure is changed. However, for the circumstance of dipole array antennas with the same period of 250 µm, its properties were severely affected by the flip chip structure. These results revealed that the double-slot antennas are more applicable to THz array detectors compared with the dipole antennas, and can more effectively reduce the influence of the flip chip. Furthermore, we integrated the double-slot antennas with Nb5N6 THz array detectors using the micro-fabrication technology. The measured results showed that the maximum responsivity of Nb5N6 THz array detectors at 0.64 THz was 312 V/W. This indicates that the double-slot antennas possess the advantages of facile preparation and large-scale integration, and have a great prospect in flip chip packaging of THz array detectors. In addition, the high sensitive THz cameras can be realized in future by this kind of double-slot antenna-coupled THz array detectors integrated with COMS readout circuits.
Funding
National Natural Science Foundation of China (11227904, 61521001, 61801209); Key Technologies Research and Development Program (2018YFB180005); Nanjing University; Priority Academic Program Development of Jiangsu Higher Education Institutions; Fundamental Research Funds for the Central Universities; Jiangsu Key Laboratory of Advanced Techniques for Manipulating Electromagnetic Waves.
Disclosures
The authors declare no conflicts of interest.
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