2002 grd International Conference on Microwave and Millimeter Wave Technology Proceedings

High-Performance Microwave Passive Components on Silicon Substrate

Kevin J. ChenS, Xiao Huo, Lydia L. W. Leung, and Philip C. H. Chan

Department of Electrical and Electronic Engineering, Hong Kong University of Science and Technology Clear Water Bay, Hong Kong China

Abstract Righ-Performance microwave passive components

are demonstrated on standard silicon substrate incorporating a low-k Benzocyclobutene (BCB) layer. Metal ohmic loss and substrate coupling loss, the two major factors that degrade the on-chip passive components are suppressed by the employment of electroplated copper and the low-k BCB layer, respectively. Spiral inductors exhibit Q-factor as high as 25 at 2 GHz. A low-loss, low-pass microstrip lranrmission line based microwave fiiter has been fabricated. The filter has a cut-off frequency at 10 GAz with an insertion loss of -1.1 dB. The fabrication Drocess is low-cost and low-1emDeratui-e. makine it kitable for port-IC proces for- high perfarmake KFIC’s and MMIC’s.

1. lntroduetion

The advent of wireless and portable applications leads to an increase in the demand for low-cost and miniaturized radio-frequency (RF) and microwave inlegrated circuils. Nonetheless, some key. microwave passive components, such as spiral inductors, transmission lines and filters, when placed on the same chip with the RF and microwavc circuits, usually suffers from high loss, or low quality factor (U). This is mainly due to the severe substrate loss of the standard silicon (low resistivity, CMOS-grade) at microwave frequencies and to a lesser extent, the ohmic loss of the aluminum (AI) thin-film. For the purpose of integrating low-loss components with RF/microwave circuits on the same silicon wafer, different approaches, including creating an interfacing layer on the silicon substrate [1-2], employing micromachining techniques [3-51 and multiplayer thin film multichip modulc (MCM-D) 161 have been used to reduce the substrate loss. Although the micromachining techniques can greatly recfuce the substrate loss, the fabrication process is rather complicated and the mechanical strength of the structure may not he strong enough far praaical use.

In this paper, we report a low-cost device technology for fabricating high-performance microwave passive components on silicon substrate. This technology employs highly conductive electroplated Cu layer and spin-on BCB dielectric layer. High performance spiral inductors and transmission line based low pass filters are demonstrated.

II. Fabrication Technology

The fahrication process is illustrated i n Fig. 1 with the fabrication of a spiral inductor as the example. The fabrication of microwave transmission lines and filters are fabricated with the same process procedure. The fabrication was carried out on a CMOS-grade p-type silicon substrate with a resistivity of 20 n c m .

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Step 1

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DCB. ........................ . . . . . . . . . . . . ...... . . . ‘..3r?=, ,... .. - . . Step2 I

Step 3

Step 5

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Step6 1 Figure 1: Pmcess flow and cross-sectional schematic of the high-Q inductan: Step 1): oxide and metal ono (aluminum) patterning; 2) BCB coating and via opening; 3) TiW/Cu seed layer sputtering; 4) Thick PR coathg and patterning; 5 ) electroplaIing copper; 6 ) PR and TiWiCu seed layer removal.

Aluminium metal strips sandwiched by 0.5 pm oxide layers (both below and above) were first fabricated on the silicon substrate to simulate a CMOS wafer after the conventional AI-based IC process. The AI strips cau provide the interconnections between the inductors and the CMOS integrated circuits on the same wafer. A photosensitive BCB layer was then spin-coated and patterned. Via holes ( I S pm wide) were opened on the aluminium strips for contacts with Cu. After the curing process of thc BCB layer, a thin TiW (thickness 80 nm) and copper seed layer (thickness 350 mu) was sputtered on the silicon wafer. A thick photoresist layer was then coated on the wafer and the device (inductors and transmission lines) pattern was defined by photolithography. The copper metallization was completed using electroplating technique. The copper thickness is decided by the photoresist thickness (controlled by the spinning speed). Finally, the photoresist and

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TiW/Cu seed layers were removed by chemical stripping and etching. It should he noted that the entire inductor fabrication is carried out below 250°C, which is suitable for post-IC process.

In. High-Performance Microwave Passive Components

A) High-Q Spiral Inductor

In this work, on-chip inductors ranging from 1 to 5 nH are designed and fabricated using the technology described in section 11.

On-wafer S-parameters were measured using HP8510C network analyzer and microwave probes. Both one-port (single turn inductors) and two-port inductors (multiple turn inductors) were designed and measured. The pad-only characteristics were measured on the “open” pad pattern to extract the pad’s parasitics. The pad’s parasitics were then de- embedded from the overall inductor characteristics, and the characteristics of the de-embedded Cu-

Q-factors of the InH indu&ors with Cu thickness varying from 0.8 pm to 15 pm, were obtained from 0.1 to 16 GHz, as shown in Fig. 2. The BCB layer thickness was fixed at 6 p. For all four different Cu thicknesses, the Q-factor reaches peak value around 2GHz. The inductor with 15 pm thick Cu reaches a maximum value of 25. The Q-factors show little difference beyond 10 GHz, indicating that the substrate loss becomes the dominant loss mechanism at higher frequencies.

inductor were obtained. . ’. \

Q4 -.

Q1: adllhrm QkClFLhrm Q1: ClFmml w: *EUm

0 2 4 6 8 10 12 14 16

Fresusncy (m) Figure 2: Comparison of Q-factors for I-nH inductors with different copper thickness. The thickness of the copper layer is 6 mm.

The use of low-k BCB dielectric reduces the coupling between the inductor and the lossy silicon substrate, and therefore enhances the Q-factor. As the BCB layer thickness increases, the frequency a1 which the substrate lose becomes significant also

increases, resulting in higher Q-factor at higher frequency. Figure 3 shows the Q-factor of the InH inductors with 6- and 12-um BCB. The copper thickness for these inductors is fixed at 6um. Substrate effect is suppressed for the inductor with thick BCB. The improvement becomes more significant for frequencies higher than 2GHz.

35

30

8 25 z 20

f 15 a a 10

5

0 0 2 4 6 8 1 0 1 2

Frequency (GHz)

Figure 3: Compa%on of Q-factors for I-nH inductors with different BCB thickness. The thckness of copper layer is 6um.

0 1 2 3 4 5 6 7

Freqency (GHZ)

Figure 4: Quality factors for inductors of different inductance values with the same outer dimension on the Same wafer. (Cu=hum, RCB=hum)

Since the inductor-substrate coupling is proportional to the planar area of the inductor, larger inductors will have stronger coupling effect due to its larger number of turns and periphery. The Qfactor of two different inductors (3.2 and 5.2 nH, respectively) is plotted in Fig. 4. The copper and BCB are both 6p- th ick . The 3.2nH one has a smaller area compared to the 5.2 nH one and therefore has a higher quality factor.

An equivalent circuit, shown in Figure 5, was used to model the high-Q on-chip inductors. Ls, Cs, and Rs account for the inductance, the coupling capacitance between spiral lines, and the series resistance of the inductor, respectively. Rs is frequency-dependent (as a result of the frequency- dependent skin depth). Cox accounts for the coupling capacitance between the metal (Cu) and

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the lossy silicon substrate. Csi and Rsi account for the substrate capacitance and resistance.

0 2 4 6 Frequency (GHz)

Figure 5 : Physical model for two-port inductors

Two-port model extraction procedure was carried out to obtain the model parameters [7-81. Figure 6(a) and 6(b) show extraction results (Magnitude and Phase) of SI, and SI? for a 5.2 nH inductor with Gum copper and 6um BCB. Excellent agreement between the measured and simulated S- parameters was obtained.

B) Equal-Ripple Low-Pass Filter

A lOGHz low-pass filter is designed using microstrip lines. For a third order 3-dB equal-ripple low-pass filter, the normalized low-pass prototype element values [9] are gl =1.5963. g2 =1.0967, g3 = 1.5963 and g4 = 1 .0000 , which is shown in Figure . After using Richard's transformation to convert series inductors to series stub and shunt capacitors to shunt stubs. All stubs are A/S long at resonant frequency, mc. As series stub is difficult to he implemented, Kuroda identities are used to convert the serics stubs to shunt stubs as shown in Fig. 7 and the dimensions and characteristics are

'summarized in Fig. 8 and Table 1. With the dielectric thickness, H, equals to 6pni and copper thickness, T, equals to 6pm, the highest impedance that can he achieved is 117R. Hence, the high- impedance sections are chosen to he 117R instead of 1 3 0 0 .

The on-wafer S-parameter measurement results of a 50R microstrip line is shown in Fig. 9. The transmission loss of the microstrip line at 10 GHz is about O.BdBlcm, much lower than that of the conventional transmission line on silicon substrate, which is ahout 4dBimm in 40Q-cm silicon substrate [IO-111. The much improved loss characteristic is owing to the low-k BCB dielectric between the microstrip lines and AI ground plane, with the absence of the lossy silicon substrate. The simulation and measurement results of the IOGHz low-pass filter are shown in Fig. 10. The results agree well with each other. Below lOGHz, the loss is oily 1.1 dB, while at 1 IGHz, the loss is 4.5 dB. The return loss of the filter is -18.1 at 10 GHz and below -10 dB withm the operating range (100 MHz to I O GHz).

0-0 0 2 4 6

Frequency (GHz)

(b) l.* 1

d 0.8

'50.6

0.4

. 0.2

0

- 0 -10

-30 5 -40 0 -50 3 -60 -70 -80

. . . . . . . . . -20 T

~ ~ . . . . .

. . . . . . . . . .

0 2 4 6 Frequency (GHz)

Figure 6 Model Extraction results of two-port S- parameters (a) (S11) magnitude and phase. @) (S12) magnitude and phase. Symbols denote the measured results while the lines indicate the simulated results.

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Pime 7: Lumped-element low-pass fiter prototype. .>, ;.i*::,

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Impedance, &(Cl) I Width (p) I €.I ACKNOLDGMENT: This work is supported by A < C I 17Q7 7 I6 RGC Earmarked Grant HKUST6025197E. _,.U

50 81.3 117

.,.“a 15.2 2.13 HKUST6130/00E and HKUSTDAG01102.EG34. 6.06 1.98 2.51 1.87 References:

Integrated CircuiD&mposium, p. 125-1%. ‘ r31 Jun-Bo Yoon. Chui-Hi Han. Euisik Yoon and _ .

Choong-Ki Kim. “Monolithic high-Q overhang inductors fabricated on silicon and glass substrates. ‘I 1999 International Electron Devices Meeting. Technical Digest, pp.753- 74L ,_I”. .,, _ _ _ .- .. .I-_c_ ..1

P 6 i o 16 1) [4] K. Takahashi, M.Matsuo and h. Ogura, “A F-n”rY,“G,*l Compact V-Band FiltedAntenna Integrated

Receiver IC Built on Si-Micromachined BCB Suspended Structure”, IEICE Trans. Electron., Vol. E84-C, No.10.. p.1506-1514, Oct 2001.

Figure 9: Insertion Loss ofthe 5OQ microstrip line.

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Figure IO: Simulation and Measurement results of the lOGHz low-pass filter.

IV. CONCLUSION

High-performance microwave passive components such as high-Q on-chip spiral inductors and low-loss low-pass filters have been demonstrated on standard silicon substrate by incorporating the electroplating technique and the spin-on low-k BCB dielectric layer. As the whole fabrication process can be completed bellow 250°C these passive components can he integrated with other RF devices for RFIC’s and MMICs’ implementation through a post-IC process.

[SI P. Blondy, A. R. Brown and G. M. Rebeiz, “Low-loss Micromachined Filters for Millimeter-Wave Communication Systems”, IEEE Trans. On Microwave theory and Techniques, Dec. 1998, p.2283-2288.

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