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Microwave Band-pass Filter and Passive Devices Using Copper Metal Process on Al2O3 Substrate
Nowadays the portable electronic appliances, such as laptop computers, Personal Digital Assistants (PDAs), cellular phones, MP3 players, etc., are rapidly growing in the life electrical popularization. Generally, the hand-held products demand miniaturization and multi-functionality, which are achieved by the module IC technology with highly integrated package. The high integration of passive components is a very important issue, and those devices occupy about 70 % space of the whole IC area . Therefore, the high-Q inductors are quite necessary for the MMIC (Monolithic Microwave IC). They can not only reduce the chip size by eliminating discrete components, but also degrade the parasitic effect by wire-bonding issue.
In this report, we explored that the characterization of passive components, the spiral inductors and MlM capacitors on A1203 substrates. In the meanwhile, an experimental investigation was undertaken to characterize the Q-factor of spiral inductors, which were carried out with different metals, Gold (Au) and Copper (Cu). In addition to the equivalent circuit models of these components, the coupled resonating band-pass filter was designed and fabricated. Finally, we designed the band-pass filters operating at the frequency of 5.2 GHz, which was the most frequently adopted in the current commercial wireless communication system [1, 2].
In the beginning, the passive components should be characterized, and the equivalent parameters model of these components must be built. The investigation of the Q-factor characterization was executed by spiral inductors with variable turns on the high-resistivity A1203 substrate. Then, we simulated S-parameters of the coupled resonating filter by the Advanced Design System (ADS), and particularly focused on the filter design at 5.2 GHz [1, 2]. Finally, the 5.2 GHz coupled resonating band-pass filters were fabricated and S-parameters were measured by the Vector Network Analyzer. The results obtained from S-parameters would provide the equivalent circuit models of spiral inductors and inter-digital capacitors for band-pass filter design.
Equivalent Circuit Parameters of Spiral Inductors and Inter-digital Capacitors
The inductor is one of the important passive devices which determine the performance of microwave integrate circuit; in particular, the performances of inductors are critical in the microwave circuits. Therefore, the Q-factor is the significant characterization parameter, and indicates a figure-of-merit of the inductor quality. The Q-factor is limited by the metal conductor loss. However, the conductor loss can be depressed by thickening or widening the conductor metal, which can improve the Q-factor of this inductor. From the substrate point, the Alumina (A1203) substrate with high dielectric constant(εr ≒10) and low impedance, can reduce the parasitical capacitor in the signal transmission [3,4]. Nevertheless, the parasitical effect causes the high transmission loss and low self-resonant frequency. This structure includes the SiO2 (εr ≒3.9) layer sandwiched between the 2μm-thickness copper metal and the 0.2μm-thickness gold metal on the Alumina A1203 substrate, as depicted in Fig.1. Then, we simulated S-parameters of the spiral inductors and inter-digital capacitors by the ADS, and the effect of the electric-magnetic with passive device layout computed by the HP-Momentum.
Fig. 1 Schematic layer structure of the spiral inductor fabricated on the A1203 substrate.
For the main fabrication of the spiral inductor integrated on the A1203, the first-level metal with 0.2 μm thickness was evaporated the substrate by Au metal in high vacuum chamber system. After the Au metal was formed, a SiO2 dielectric layer with 0.27 μm thickness was also evaporated on the substrate. During this procedure, the SiO2 via process, via-hole connecting first-level metal and second-level metal, was performed by the dry etching. Then, the top copper metal with 2μm thickness was formed by electroplating technology. Finally, the spiral inductor on the A1203 substrate was achieved, and the microphotograph was shown in the Fig. 2.
Fig. 2 Microphotograph of the spiral inductor fabricated on the A1203 substrate.
The Fig. 3 depicts the equivalent circuit models of spiral inductors and inter-digital capacitors [5, 6]. The equivalent circuit models of inductors and capacitors were evaluated for the measured two-port S-parameters. First, the two-port S-parameters were transferred into one-port S-parameters. Then, the two-parameters could be calculated. Based on the real and imaginary parts of Z-parameters transferred from the measured two-port S-parameters, the effective resistance (the real part) and the effective inductance (the imaginary part) in the Z-parameters versus frequencies could be extracted . Finally, the equivalent circuit parameters of spiral inductors and inter-digital capacitors were summarized in Tab.1.
Fig. 3 (a) Equivalent circuit model of spiral inductor. (b) Equivalent circuit model of inter-digital capacitor
Tab.I (a). The equivalent circuit parameters of spiral inductors. (b). The equivalent circuit parameters of inter-digital capacitors.
Characterization of Spiral Inductors with the Au and Cu metal
Choosing the metal with high electric conductivity for the high-Q of the inductor is necessary and can improve metal losses the constant of decaying for signal transmission. Hence, we fabricated the spiral inductors by the different metals, Gold and Copper, respectively. The inductance of inductor verse spiral turns was displayed in Fig.4, which the curves trend consisted of the Gold and Copper metals.
Fig. 4 The inductance of spiral inductor with Au, Cu metal.
Fig.5 illustrated the parasitic resistance of inductor for the Gold and Copper metals, and exposed the parasitic resistance was reduced by the Copper metal. Actually, the electrical conductivity of the Copper is larger than that of the Gold (Cu =5.961 x 107 /ohm‧m , Au =4.521 x 107 /ohm‧m ). For the parasitic resistance issue, we compared the parasitic resistance of two-half turns inductor with the Cu and Au metals. The equivalent circuit parameters were shown in Tab. II, which revealed the small parasitic effect with the Copper metal.
Fig. 5 The parasitic resistance of inductor for the Gold and Copper metals.
Tab.II Compared the parasitic resistance of two-half turns inductor with the Cu and Au metals.
Furthermore, the Q-factor of inductor verse frequencies was shown in Fig. 6, which depicted that the Copper metal could diminish the parasitic resistance of inductor and improve the Q-factor in the spiral inductor. Tab. III described the maximum Q-factor (Qmax) of variable turns, and showed the high Qmax with the Copper metal. Therefore, the fabrication of Copper metal was demonstrated, and obtained that high quality inductor in this experiment.
Fig. 6 Comparison of the Q-factor with Au and Cu metals.
Tab.III The maximum Q-factor (Qmax) for variable turns inductor with the Cu and Au metals.
Design and Measured 5.2 GHz Band-Pass Filter
This 5.2GHz band-pass filter was achieved by a pair of resonators (L1 and C3) with a coupling capacitor (C2), as shown in Fig. 7 [4, 8, 9]. These resonators were executed by the LC tank circuits, which were used the equivalent circuit model of spiral inductor and MIM capacitor in Tab. I. Moreover, the input and output capacitors (C1 and C3) were performed for the DC blocking and matching in the band-pass filter circuit [8, 9]. Finally, the 5.2GHz band-pass filter with the bandwidth of 2.3 GHz, from 4.4 GHz to 6.7GHz, presented the insertion loss (S21) of -2.5 dB and the return loss (S11) of -29.1 dB, as shown in the Tab. IV.
Fig. 7 The schematic layout of the 5.2GHz band-pass filter.
Tab. IV Summarized the S-parameters by the simulation and measurement.
The microphotograph of the band-pass filter with a chip area of 811μm x 678 μm was depicted in the Fig.8. The S-parameters of this 5.2 GHz band-pass filter were measured by Vector Network Analyzer. In the Tab. IV, we summarized the S-parameters of the simulation and measurement. It described that the central frequency was 5.1 GHz with the bandwidth of 2.8 GHz in the simulation, and the in-band insertion loss and off-band return loss were -5.8 dB and -27 dB in the measurement, respectively.
Fig. 8 The microhotograph of 5.2 GHz bandpass filters by MIM capacitors ( Dimension : 811μm x 678 μm )
In addition, the insertion loss (S21) of this band-pass filter were simulated and measured, as shown in Fig. 9. The Fig. 9 indicated that the insertion loss (S21) of the measurement was slightly shifted comparing with that of the simulation, which the return loss of upper band was degraded in the measurement. The central frequency was shifted, since the grounding effect was neglected, that was, the via-hole ground did not work in our experiment. Moreover, the fabrication variation of the inter-digital capacitors might shift the center frequency. However, these S-parameters revealed the low insertion loss and high return loss. Therefore, the 5.2 GHz band-pass filter should provide for the Monolithic Microwave Integrated Circuits applications.
Fig.9 Simulated and measured microwave performance of 5.2 GHz bandpass filters
The fabricated spiral inductors with variable turns could be realized, and obtained the high quality-factor on high-resistivity Al2O3 substrates. The passive components were stimulated by EM simulation, and the measured data with the equivalent circuit models were also extracted. The Cu metal interconnection with an extremely low electrical resistivity (1.7 × 10-8 Ω‧m) was also employed, improve the signal loss in the communication transmission circuit. In short, the 5.2 GHz band-pass filter was performed by these L-C components. It demonstrated that the accuracy models of passive devices exhibited the low insertion loss and high return loss in the 5.2GHz band-pass filter, which the characterization should be adopted for the MMIC applications.
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