# Fabrication of QFN-Packaged Miniaturized GaAs-Based Bandpass Filter with Intertwined Inductors and Dendritic Capacitor

^{*}

^{†}

## Abstract

**:**

_{0}(0.8 × 0.8 mm

^{2}) for the BPF. The QFN-packaged GaAs-based bandpass filter was used to protect the device from moisture and achieve good thermal and electrical performances. An equivalent circuit was modeled to analyze the BPF. A description of the manufacturing process is presented to elucidate the physical structure of the IPD-based BPF. Measurements were performed on the proposed single band BPF using a center frequency of 2.21 GHz (return loss of 26.45 dB) and a 3-dB fractional bandwidth (FBW) of 71.94% (insertion loss of 0.38 dB). The transmission zero is located at the 6.38 GHz with a restraint of 30.55 dB. The manufactured IPD-based BPF can play an excellent role in various S-band applications, such as a repeater, satellite communication, and radar, owing to its miniaturized chip size and high performance.

## 1. Introduction

## 2. Design and Fabrication

#### 2.1. Design and Circuit Analysis

_{0}of an LC circuit can be calculated as follows:

_{x}passivation layer and GaAs substrate, and the center capacitor. To accurately evaluate the equivalent circuit, we developed an analytical model for performance evaluation of the proposed BPF using the segmentation method, the mutual inductance method, and the simulated scattering parameters (S-parameters) [18,19].

#### 2.1.1. Model inside the Segment Box

_{T}and Lt represent the series resistance and series inductance of this segment, respectively, C

_{SiNx}represents the capacitance associated with SiN

_{x}, and C

_{SUB}and G

_{SUB}represent the capacitance and conductance associated with GaAs, respectively.

_{eff}was employed to model the equivalent circuit, which can be calculated as follows:

_{1}and c

_{2}are the fitting parameters to match the resistance and inductance with the measurement results, respectively. The resistance of each metal line (R-line) can be obtained using W

_{eff}as given in Equation (4) to evaluate the signal losses [20],

_{eff}[20]:

#### 2.1.2. Models outside the Segment Box

_{i_j}is the coupling capacitance between the adjacent metal tracks, where i and j are the number of the segments marked in Figure 2a. The capacitor is calculated using the following Equation [21]:

_{0}is the vacuum permittivity, and Q and V are the potentials between the normalized charge and the center position of the two ends, respectively. In addition, the capacitive effect of the four air bridge structures as a result of the air gap between the leads and bond layers cannot be neglected and is expressed as follows [21]:

#### 2.1.3. Embedded Capacitor

_{C}, the resistor R

_{C}, and the inductor L

_{C}are introduced to construct the model of the embedded center dendritic capacitor, taking the inductive effect and ohmic loss into consideration. However, the complex structure of the center capacitor results in difficulties in calculating the above parameters. Therefore, the simulated S-parameters and Y-parameters are introduced to calculate the capacitance, resistance, and inductance as given in Equations (9)–(11) [21]. In addition, the capacitance of the center capacitor can be tuned to match the design target by optimizing the size. The frequency-dependent simulation results of the capacitance, resistance, and inductance of the optimized center capacitor are shown in Figure 3.

#### 2.1.4. Substrate-Related Parasitic Effects

_{SiNx}and C

_{SiNx_AB}), the conductance related to the substrate (G

_{SUB}and G

_{SUB_AB}), and the capacitance related to the substrate (C

_{SUB}and C

_{SUB_AB}). The losses caused by eddy currents are negligible since the resistivity of the GaAs substrate is greater than 10 Ω·cm [22]. The above six parameters can be calculated as given in Equations (12)–(17):

_{AB}subscripted components represent the elements related to the air-bridge area, which can be considered as frequency-independent as their lengths are small. In Equations (14)–(16), ε

_{0}, ε

_{GaAs}, and ε

_{SiNx}are the permittivity in free space, of the substrate, and of the passivation layer, respectively, and t

_{GaAs}and t

_{SiNx}are used to indicate the thicknesses of the substrate and passivation layer, respectively. Therefore, a new function F (W

_{eff}, t) is introduced with respect to the effective linewidth, metal thickness, and the frequency-dependent permittivity ε

_{eff}(f) to estimate C

_{SiNx}, C

_{SUB}, and G

_{SUB}[23].

#### 2.2. Fabrication

_{6}/Ar plasma with an inductively coupled plasma (ICP) etcher, as illustrated in steps 10–12. In step 13, the positive PR was coated again and then exposed using mask 3 to prepare for the second metal layer deposition. Then, the second metal layer of Cu/Au was formed with a thickness of 1.8 μm using electroplating technology, as given in step 15. In step 16, a negative PR, instead of a positive PR, was subsequently coated and then exposed using mask 4 to define the pattern of the third metal layer. Then, in step 17, the third metal layer was electroplated using Cu/Au with a thickness of 4.5/0.5 μm, which is the same as the first metal layer. In step 19, the air-bridge structure was formed, and the PR was removed using acetone solution and wet etching technology. At the same time, other normal three-metal layer structure-like intertwined coils and the center capacitor were also produced. Then, a 300 nm SiN

_{x}passivation layer was deposited using PECVD to protect the device from moisture and oxidation, as given in step 20. Subsequently, a positive PR was coated and then exposed using mask 5 to etch the passivation layer via the ICP dry etch process with SF

_{6}/O

_{2}to connect the contacts for further performance testing, as illustrated in steps 21–22. Finally, in step 24, polishing, dicing, and wire-bonding processes were performed to mount the BPF device on the PCB, such that the RF performance of the fabricated device can be measured. The manufacturing techniques used in the IPDs process are explained in detail in Table 1.

## 3. Results and Discussion

^{3}), the complete PCB (2 × 2 cm

^{2}), and the packaged chip are shown in Figure 6b–d, respectively. The overall dimensions of the pattern are 0.8 × 0.8 mm

^{2}, and the scanning electron microscope (SEM) images were taken to clearly illustrate the manufactured pattern, as shown in Figure 6e. The enlarged air-bridge area and cross-section of the three metal layers are shown in Figure 6f.

## 4. Conclusions

_{0}× 0.021 λ

_{0}(0.8 × 0.8 mm

^{2}). The measured results show a relatively good consistency with theoretical prediction and simulation, with a low insertion loss of 0.38 dB, and an ultrawide 3-dB FBW of 71.94%. The proposed BPF can be employed in a modern communication system owing to its high performance and miniaturized size as a result of the GaAs-based IPD fabrication technology. However, the lifecycle and compatibility of the proposed BPF in practical application were not investigated in this study; furthermore, its selectivity is limited by it as a low-order device, and its sensitivity will be verified in our future research to promote the practical application of the IPD technology.

## Author Contributions

## Funding

## Conflicts of Interest

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**Figure 1.**Design of the proposed bandpass filter (BPF): (

**a**) 3D view of the packaged BPF on a printed circuit board (PCB) board; (

**b**) enlarged view; (

**c**) side view; (

**d**) three metal layers (leads, text, and bond).

**Figure 2.**Equipment circuit model of the proposed BPF: (

**a**) number of segment boxes of the proposed BPF; (

**b**) π-type lumped-element model inside the segment box; (

**c**) equivalent circuit model of the proposed BPF.

**Figure 3.**Simulation results of the (

**a**) capacitance, (

**b**) inductance, and (

**c**) resistance of the optimized center capacitor.

**Figure 4.**(

**a**) S

_{11}and S

_{21}of the circuit model and full-wave simulations. (

**b**) Current density at 2.2 and 5 GHz.

**Figure 6.**S-parameter measurement setup of the proposed IPD-based BPF with enlarged views of different parts. (

**a**) Measurement setup. (

**b**) BPF installed on an aluminum cube. (

**c**) Top view of the entire PCB. (

**d**) Top view of the packaged chip on the PCB. (

**e**) Top view of SEM image. (

**f**) Enlarged air-bridge area and cross-section of the three metal layers.

Fabrication Objective | Technique | Material |
---|---|---|

Passivation layer | PECVD | SiNx |

Photoresistor | Spin-coating | Negative/positive PR |

PR removal | Lift-off | Acetone |

Seed metal | Sputtering | Ti/Au |

Metal layer | Electroplating | Cu/Au |

Via | ICP etching | SF_{6}/O_{6} |

References | Technology | Fractional Band Width (%) | Passband (GHz) | Insertion Loss (dB) | Return Loss (dB) | Circuit Area |
---|---|---|---|---|---|---|

[31] | Silicon IPD | 107.63 (3 dB) | 6.5 | 1.1 | 15 | 0.219 λ_{0} × 0.181 λ_{0} |

[32] | Silicon IPD | 33.33 (3 dB) | 2.4 | 2.3 | 10 | 0.024 λ_{0} × 0.024 λ_{0} |

[33] | Silicon IPD | 16 (10 dB) | 1.7 | 2.56 | 12 | 0.039 λ_{0} × 0.037 λ_{0} |

[34] | Glass IPD | 49.62 (3 dB) | 2.6 | 0.6 | 30 | 0.018 λ_{0} × 0.009 λ_{0} |

[35] | Glass IPD | 36.73 (10 dB) | 2.1 | 3.2 | 22 | 0.019 λ_{0} × 0.019 λ_{0} |

This work | GaAs IPD | 71.94 (3 dB) | 2.21 | 0.38 | 26.45 | 0.021 λ_{0} × 0.021 λ_{0}(0.8 × 0.8 mm ^{2}) |

References | Technology | Fractional Band Width (%) | Insertion Loss (dB) | Return Loss (dB) | Passband (GHz) | Circuit Area |
---|---|---|---|---|---|---|

[36] | Microstrip | 13.3 | 1.1 | > 20 | 0.975 | 0.094 λ_{0} × 0.08 λ_{0} |

[37] | HTS | 2.6/2.4 | 0.18/0.32 | > 16 | 1.9/2.6 | 0.182 λ_{0} × 0.156 λ_{0} |

[38] | HTCC | 5.5 | 1.8 | > 15 | 2.25 | 6.9 × 39.9 mm^{2} |

[39] | LTCC | 12.5 | 2.4 | 15 | 2.4 | 0.058 λ_{0} × 0.058 λ_{0} |

This work | GaAs IPD | 71.94 | 0.38 | 26.45 | 2.21 | 0.021 λ_{0} × 0.021 λ_{0}(0.8 × 0.8 mm ^{2}) |

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## Share and Cite

**MDPI and ACS Style**

Chen, J.; Wang, Z.-J.; Zhu, B.-H.; Kim, E.-S.; Kim, N.-Y.
Fabrication of QFN-Packaged Miniaturized GaAs-Based Bandpass Filter with Intertwined Inductors and Dendritic Capacitor. *Materials* **2020**, *13*, 1932.
https://doi.org/10.3390/ma13081932

**AMA Style**

Chen J, Wang Z-J, Zhu B-H, Kim E-S, Kim N-Y.
Fabrication of QFN-Packaged Miniaturized GaAs-Based Bandpass Filter with Intertwined Inductors and Dendritic Capacitor. *Materials*. 2020; 13(8):1932.
https://doi.org/10.3390/ma13081932

**Chicago/Turabian Style**

Chen, Jian, Zhi-Ji Wang, Bao-Hua Zhu, Eun-Seong Kim, and Nam-Young Kim.
2020. "Fabrication of QFN-Packaged Miniaturized GaAs-Based Bandpass Filter with Intertwined Inductors and Dendritic Capacitor" *Materials* 13, no. 8: 1932.
https://doi.org/10.3390/ma13081932