CHAPTER 1 INTRODUCTION
1.2 S ILICON -B ASED P ASSIVE C OMPONENTS
Fig. 1-8 R&D cost for silicon processes [Morris Chang, “Foundry Future: Challenges in the 21st Century”, in ISSCC 2007].
(a) (b) (c)
Fig. 1-9 Silicon-based system-on-chips (a)Atheros[JSSC2004] 0.25um (RF:23mm2) (b)Atheros[ISSCC 2006] 0.18um (RF:13.5mm2) (c)DICE[JSSC2006] 90nm (RF:12mm2).
1.2 S ILICON -B ASED P ASSIVE C OMPONENTS
The era of the wireless applications with high data-rate transmission and multiple functions is coming, e.g., the IEEE 802.11a/b/g combo system [1], ultra-wideband (UWB) system [2], and WiMAX system [3]. The range of carrier frequencies and their bandwidth constantly increase. The obligation of the complicated data processing belongs to the baseband design, while the RF integrated circuit (IC) design takes responsibility for the wide range frequency and broad bandwidth operation.
Nevertheless, the design of the high-frequency and wideband RF circuits is a big challenge in the overall solution implementation. For an active mixer, the transistors have natural instinct to perform wide range and broad bandwidth frequency translation. Due to the input/output matching networks, narrowband passive
Tech. Cost (Wafer/Mask set) 0.18m 1.4K/120K 0.13m 1.8K/300K
90 nm 2.5K/900K
64 nm 3.8K/1400K
Introduction
6
components, and loading effects, the mixer’s wideband ability is restricted. Hence, microwave broadband passive components, such as Marchand baluns and rat-race couplers, can be integrated into ICs for wideband applications.
The silicon radio-frequency/microwave integrated circuit revolution has brought inductors and transformers into integrated circuits. As the silicon circuit reaches the millimeter-wave regime, new circuit design concepts arise. In our researches, a
CR-LR quadrature generator takes the place of a polyphase filter while inductors and
transformers along with parasitic capacitance operate as a transmission line and a coupled line, respectively, as shown in Fig. 1-10. The inductor absorbing parasitic capacitors to form a transmission line enables the high frequency operation. It is now feasible to integrate passive components based on quarter-wavelength transmission lines in the integrated circuits.C
L C
R R
C R
R 0
90
90
0
R L C
V
IN VAVB
Fig. 1-10 New design concepts for high-frequency applications.
Silicon-based technologies have the properties of high integration and low-cost production. Active devices have advanced in possession of the cut-off frequency of more than 100 GHz, and they are suitable for microwave and millimeter-wave applications [4]. Today, passive components are largely implemented using silicon-based technologies in the RFICs. For example, inductors and transformers are
1.2 Silicon-Based Passive Components
applied in oscillators, low-noise amplifiers, and mixers [5]. However, the standard silicon substrate with a resistivity of approximately 10 ·cm deteriorates signals, causes crosstalk between two adjacent passives, and influences the functions of passive components. Thus, it is a big challenge to form useful RF passive components on a silicon substrate for microwave and millimeter-wave applications [6], [7].
To reduce the substrate loss, many solutions have been developed, such as micromachining, high-resistivity substrate, silicon-on-insulator (SOI) process and shielding. Abidi et al. applied the front-side micromachining to form a suspended inductor in a CMOS RF amplifier. Thus, this inductor has higher self-resonance frequency and quality factors and can work at higher frequencies [8]. The backside micromachining is also employed to eliminate the unpredictable pattern-dependent etching behavior of front-side micromachining [9], [10]. Besides this, the high-resistivity (~1 k·cm) silicon substrate is also employed, but the entire substrate with high resistivity is not compatible with active devices, due to the latch-up issue [11]. The local high-resistivity substrate can be obtained by proton-damaged ion implantation with high energy (15 MeV) before the backend process, or with low energy (~4 MeV) after the backend process [12]. The substrate resistivity increases significantly to ~1 M·cm. The performances of the transmission lines and spiral inductors on the proton-damaged high-resistivity silicon substrate are enhanced [12], [13]. Shielding, like patterned ground shields [14] and floating shields [7], is inserted between the signal path and the silicon substrate, to resist the substrate loss without any extra process. This method is adequate for the process with multiple metal layers.
In this dissertation, our passive components function well in standard silicon-based IC processes because of the new design concepts such as balanced path loss and distortionless transmission lines. The implementation directly on the silicon
Introduction
8
substrate is good for size compactness thanks to the high dielectric constant. Meander lines compress the size while stepped-impedance and lumped-element techniques shrink the transmission-line length further as well as reducing the loss. In this dissertation, it shows that passive components such as CR-LR reactive quadrature generators, ring hybrids, and Marchand baluns could be successfully integrated into standard silicon-based ICs.
As shown in Fig. 1-10, the proposed reactive quadrature generator is derived from the conventional C-R and R-C sections. The low-pass session, R-C section, is substituted by the L-R network. The truly balanced quadrature signals are generated when R=ω0
L=(ω
0C)
-1. Regardless of frequencies, the input impedance equals R and the output signals are always in quadrature. When compared with a polyphase filter, this reactive quadrature generator has three advantages. 1) The generator has low loss thanks to the absence of resistors and is suitable for high-frequency applications. 2) The input impedance is constant and the wideband matching is achievable. 3) A multi-band quadrature generator can be established by increasing the order of the L-C networks.A wideband phase-inverter rat-race coupler and a broadband Marchand balun are presented to demonstrate the new design concept of the transmission lines and coupled lines for silicon-based millimeter-wave ICs, respectively. Our microwave passives are directly implemented on a lossy silicon substrate. The transmission-line length of passive components reduces with operation frequency increase and so does the loss, as shown in Fig. 1-11. Therefore, the rat-race coupler and Marchand balun operate better at millimeter-wave frequencies. The passive components and the proposed design approaches can be applied for the millimeter-wave regime.