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Abstract
A compact millimeter-wave on-chip sensor for dielectric detection is presented using gallium arsenide technology based on spoof surface plasmon polaritons (SSPPs). The proposed structure is developed from traditional half-mode substrate integrated waveguide (HMSIW) and its dispersion characteristics is analyzed through electromagnetic simulations. Consequently, the operating frequency and bandwidth of the on-chip sensor can be easily adjusted, which provides more flexibility for the practical application of the sensor. The linear sensing for relative dielectric constant of the film materials is acquired, with thickness-insensitive property. Moreover, the low coupling to the nearby components can be achieved due to the strong field confinement characteristics of the SSPPs, which is of great significance in the application scenarios of on-chip integrated circuits for the suppression of electromagnetic interference.
© 2023 Optica Publishing Group under the terms of the Optica Open Access Publishing Agreement
1. Introduction
Low-cost and non-invasive microwave/millimeter-wave sensors can be utilized in vast areas, such as biomedical sensing, healthcare monitoring and automatic driving [1–4]. With the interpretation of the microwave signal, the sensors could provide rich information for electronic systems [5,6]. In recent years, many techniques were utilized to develop sensors with good performance [7–9]. Nevertheless, microwave and millimeter-wave sensing still suffers from lower sensing sensitivity and challenges of size reduction, compared with other solutions such as optical and chemical ones.
To tackle the challenges, spoof surface plasmon polaritons (SSPPs) are applied to enhance the sensing sensitivity [10,11]. The SSPPs are developed from the conception of surface plasmon polaritons (SPPs) existing at optical frequencies. Different from the SPPs, the SSPPs can be deployed in lower frequency regimes (i.e., microwaves and millimeter-waves) but having the same advantages as SPPs, such as strong field confinement and enhancement due to localized surface plasmons. Thus, in microwave and millimeter-wave regimes, various SSPP components and circuits are designed and demonstrated, including the transmission lines [12,13], filters [14–18], power dividers [19,20] and antennas [21–23]. Because the SSPP propagation strongly confines the electromagnetic energy around the waveguide structure with localized field enhancement, the effect of interaction between the analytes and the electromagnetic wave is more significant if the SSPPs are employed for the sensor design [24], which means higher sensitivity of the sensor can be obtained. Therefore, SSPPs are very suitable for the enhancement of the sensing sensitivity, and some SSPPs based sensors have been developed [25–28]. However, most of these SSPPs-based sensors are designed based on the printed circuit board technique with bulky sizes.
In recent years, the techniques of integrated circuits (ICs) have been adopted in the development of sensors and other devices [29,30]. With the development of the ICs, the bulky and discrete instruments of the traditional sensing methods are replaced by the compact integrated chips, and the whole size of the sensors can be remarkably reduced. This feature facilitates the deployment of the advanced sensing technologies which brings convenience to human life.
In this paper, a compact gallium arsenide (GaAs) based SSPP sensor is proposed with enhanced sensing sensitivity, where the cross-sectional view of the GaAs process technology is demonstrated in Fig. 1. Dispersion relations are analyzed and explored with the help from electromagnetic simulations. To validate the proposed idea, a GaAs-based prototype is fabricated and measured, where measurement results agree well with simulation ones, and the linear sensing for relative dielectric constant of the film materials with low-coupling feature is achieved.
Fig. 1. Stack-up of the commercial 0.15-µm GaAs pHEMT technology (Not to scale). The relative dielectric constants of GaAs and polyimide are 12.9 and 2.9, respectively. Two metallic layers M1 and M2 are employed for the structure design of on-chip devices.
2. SSPPs based sensor
The SSPPs unit cell evolution from traditional half-mode substrate integrated waveguide (HMSIW) to the proposed SSPPs structure is illustrated in Fig. 2. As shown in Fig. 2(a) and 2(b), the conventional HMSIW and straight-slotted HMSIW structures are designed on the metallic layer M1, respectively, where the sheet resistance of the metallic layer M1 is 0.013 Ω/sq. Then, the fishbone-slotted SSPPs structure is created, where the grooves are uniformly and densely loaded on the straight slot along x-axis direction, as illustrated in Fig. 2(c).
Fig. 2. SSPPs unit cell evolution: (a) traditional HMSIW, (b) straight-slotted HMSIW, (c) proposed fishbone-slotted SSPPs. (d) Comparisons of dispersion curves, where k is the propagation constant, and (e) dispersion curve changes of the proposed SSPPs with the variance of the parameters L2 and D2. The other default parameters are set as L1 = 10 µm, L3 = 35 µm, L4 = 10 µm, L5 = 57.5 µm, D = 300 µm, and D1 = 229 µm.
The comparisons of the simulated dispersion curves among these three structures are demonstrated in Fig. 2(d), where the dimensions are set as L1 = 10 µm, L2 = 150 µm, L3 = 35 µm, L4 = 10 µm, L5 = 57.5 µm, D = 300 µm, D1 = 229 µm, D2 = 409 µm and the via size of 60 µm × 20 µm. For the traditional HMSIW (i.e., black-solid line), the dispersion illustrates highpass frequency response with the left-edge cutoff frequency at around 58 GHz. For the straight-slotted HMSIW (i.e., blue-dashed curve), the dispersion characteristic demonstrates a bending curve with the operation frequency ranging from 58 to 97.6 GHz, which represents the dispersion having bandpass frequency response. For the proposed fishbone-slotted SSPPs (i.e., red-dotted line), the dispersion characteristic also shows bandpass frequency response but with obviously smaller asymptotic frequency of 69.6 GHz than that of the straight-slotted HMSIW structure in Fig. 2(b). It indicates that the proposed structure possesses stronger field confinement ability. Note that the left-edge cutoff frequencies of these three structures in Figs. 2(a), (b) and (c) almost agree well with each other. It implies that the left-edge cutoff frequency of the bandpass filtering response is led by HMSIW structure. As shown in Fig. 2(e), the left-edge cutoff frequency can be changed by tuning the width of HMSIW (i.e., D2). Meanwhile, it is found that with the variance of the physical dimension of the grooves (i.e., L2), the right-edge cutoff frequency would be changed significantly while the left-edge cutoff frequency shows slight move. Obviously, the SSPP mode propagation evokes the high cutoff frequency. Thus, the center frequency and bandwidth of the SSPPs based sensor can be easily controlled as the physical parameters of fishbone-slotted SSPPs are changed.
Based on the proposed fishbone-slotted SSPPs, an on-chip dielectric sensor at millimeter-wave frequencies is designed as illustrated in Fig. 3(a). The structure is composed of input/output ground-signal-ground (GSG) port, transition section and several proposed fishbone-slotted SSPP unit cells. Different heights of fishbone-slotted structure in the transition section H1 and H2 are chosen for smooth momentum matching from GSG feeding to the SSPP unit cell array. Finally, the dimensions are set as: H1 = 79 µm, H2 = 149 µm, H3 = 70 µm, D1 = 229 µm, D2 = 409 µm, and L6 = 2050 µm.
Fig. 3. (a) Top view of the proposed sensor, (b) sensor with the under-test dielectric film. Side view of the sensor (c) without and (d) with the under-test dielectric film.
For sensing purpose, we directly place the dielectric film on the surface of the proposed sensor, as demonstrated in Fig. 3(b), where the part in red is the under-test film. The SSPP wave would experience different propagation environments when the sensor is loaded without and with the under-test dielectric film as illustrated in Fig. 3(c) and (d), respectively. The impedances of these two scenarios are different, which leads to different frequency responses. This feature is utilized to measure the relative dielectric constants of different films. Due to strong confinement of electric fields with localized surface plasmons, the interaction between the SSPP wave and under-test film becomes strong and the sensitivity of the sensor is improved. Besides, the proposed SSPPs based on-chip sensor is a subwavelength structure, resulting in the realization of size miniaturization.
3. Sensing ability and low-coupling feature
Figure 4(a) is the simulated and measured S-parameters of the proposed sensor without under-test film, and the inset figure is the fabricated die photograph. The vector network analyzer as well as the on-wafer GSG probing are utilized to measure the proposed on-chip sensor on CASCADE probe station. As can be seen, a bandpass filtering response is realized and its center frequency is at about 60 GHz. The operation band covers from 48.7 GHz to 71.3 GHz (i.e., the bandwidth is 22.6 GHz). Minor differences between simulation and measurement results may be due to the fabrication tolerance and measurement error.
Fig. 4. (a) Simulated and measured S-parameters for the proposed sensor without the under-test film. (b) Right-edge cutoff frequency variation of the passband against the parameter D1. (c) Left-edge cutoff frequency variation of the passband against the parameter D2. (d) Bandpass responses with different relative dielectric constants of the under-test film. (e) Bandwidth variation against the under-test film’s relative dielectric constant for the proposed sensor. (f) Bandpass responses with different thicknesses of the under-test film.
We present the proposed sensor with different frequency responses by changing the physical dimensions. As the physical parameters D1 and D2 are changed, the right-edge cutoff frequency and left-edge cutoff frequency can be independently tuned, respectively, which means that the center frequency and bandwidth of the passband can be adjusted as desired (see Fig. 4(b) and (c)). This provides more flexibility for the design of the proposed sensor.
To investigate the sensing ability of the proposed sensor, full-wave electromagnetic simulations are carried out. We place the dielectric film with a thickness of 80 µm on the surface of the sensor as illustrated in Fig. 3(d). As the relative dielectric constant of such dielectric film changes from 1 to 11 with the step of 2, the right-edge cutoff frequency decreases to smaller value gradually, while the left-edge cutoff frequency remains almost unchanged, as shown in Fig. 4(d). Thus, the 3-dB bandwidth (BW) of the frequency response against the relative dielectric constant of the under-test film is obtained, i.e., dotted line in Fig. 4(e), and then the linear fitting line is calculated accordingly. The fitting line equation can be expressed as BW = 22.93·εr − 1.11 with a regression coefficient of 0.9778. Thus a linear function can be obtained to describe the curve, i.e., solid line in Fig. 4(e). Therefore, the mapping relationship between the relative dielectric constant and the bandwidth (or the right-edge cutoff frequency) can be established. In the practical measurement, we can attain the relative dielectric constant of the under-test film by reading the bandwidth and looking up the mapping relationship. Moreover, the sensitivity against the thickness of the under-test film has been also investigated. Figure 4(f) shows the bandpass responses with different thicknesses of the under-test film when the relative dielectric constant is 9. As can be observed, the proposed sensor is insensitive to the thickness, since the bandpass responses remain almost unchanged when the thickness is tuned. It is also verified that same conclusion can be drawn as the relative dielectric constant of the under-test film is changed from 1 to 11.
For comparison, the sensor using traditional structure (i.e., the straight-slotted HMSIW in Fig. 2(b)) is built as shown in the inset of Fig. 5. The physical dimensions of the traditional structure are the same as those in Fig. 3(a). As can be observed from Fig. 5, the left-edge cutoff frequencies of the two structures are located almost at the same position, while the right-edge cutoff frequencies are different, resulting in difference of the bandwidths. Such difference is due to the different asymptotic frequency of dispersion curves as shown in Fig. 2(d). The right-edge cutoff frequency of the proposed structure is essentially lower than that of the traditional one. Although the proposed sensor structure has smaller bandwidth than that of the traditional structure in Fig. 5, the ability of coupling suppression is better than that of the traditional case, which will facilitate the integration with other circuits in the same chip to construct the sensing system.
Fig. 5. Simulated |S21| comparison between the traditional and proposed structures. The inset is the layout of the traditional structure.
Furthermore, Fig. 6 shows the electric field distribution on the surface of the SSPPs based sensor at the operating frequency 65 GHz, located within the passband. The electric field energy is mainly concentrated around the fishbone-slotted structure due to strong subwavelength confinement of SSPPs.
In order to investigate the suppression of electromagnetic interference (EMI) on the on-chip circuit, the coupling characteristic between the proposed sensor and the inductor is analyzed. In the design of ICs, the spiral inductor is one of the most sensitive passive elements to the EMI, which could lead to the deterioration of radio frequency (RF) performance. Therefore, it is important to design on-chip ICs with the consideration of the EMI suppression to the nearby inductors. Figures 7(a) and (b) show the simulation scenarios where each identical spiral inductor is closely located near the traditional and proposed sensors, respectively, with the separation distance of D3. In order to keep the comparison fair, the physical dimensions of these two cases are identical. The parameters of the inductor are selected as S1 = 28 µm, W1 = 22 µm, and W2 = 176 µm.
Fig. 7. (a) Case I; (b) Case II; (c) Comparisons of the coupling strengths (i.e., S31) with the variation of D3.
Figure 7(c) shows the simulated coupling strengths (i.e., S31) of the two cases within the passband (from 50 to 70 GHz) of the sensors. As can be seen, the coupling strength between the proposed structure and spiral inductor is always weaker than that of the traditional case, even if the separation distance D3 is varied from 300 to 500 µm. Therefore, the ability of coupling suppression of the proposed structure is better than that of the traditional case. It is because that the electromagnetic waves with localized surface plasmons are strongly confined in a limited area. Due to the suppression of electromagnetic coupling, more on-chip space can be provided for the IC design. In other words, the overall size can be effectively decreased due to low-coupling feature if the SSPPs based sensor is employed in the on-chip circuits and systems.
4. Conclusion
A fishbone-slotted SSPPs based on-chip sensor in GaAs technology has been presented, which is suitable to apply in the scenarios of on-chip sensing systems. In addition, the operating frequency and bandwidth of the proposed on-chip sensor can be flexibly controlled as desired by tuning the physical dimensions. A prototype example is fabricated and tested, which is much smaller than the SSPPs based sensors using printed circuit board technique. The proposed on-chip sensor can detect the relative dielectric constant of the dielectric film with very small size and thin thickness. The linear sensing for dielectric’s relative permittivity is realized along with ability of EMI suppression. With these advantages, the proposed sensor is of great potential for sensing applications in the on-chip circuits and systems.
Funding
NSAF Joint Fund (U2130102); State Key Laboratory of Surface Physics Open Research Project; State Key Laboratory of Advanced Optical Communication Systems and Networks (2022GZKF020); National Natural Science Foundation of China (61901354).
Disclosures
The authors declare no conflicts of interest.
Data availability
Data underlying the results presented in this paper are not publicly available at this time but may be obtained from the authors upon reasonable request.
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