A 22-GHz Push-Push CMOS Oscillator Using Micromachined Inductors

IEEE MICROWAVE AND WIRELESS COMPONENTS LETTERS, VOL. 15, NO. 12, DECEMBER 2005

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A 22-GHz Push-Push CMOS Oscillator Using Micromachined Inductors To-Po Wang, Student Member, IEEE, Ren-Chieh Liu, Hong-Yeh Chang, Member, IEEE, Liang-Hung Lu, Member, IEEE, and Huei Wang, Senior Member, IEEE

Abstract—This letter presents a low phase noise 0.35- m CMOS push-push oscillator utilizing micromachined inductors. This oscillator results in an improvement in phase noise compared with the previously published Si-based voltage-controlled oscillators (VCOs) around 20 GHz. With the high-Q inductors introduced by the micromachined structure, the oscillator achieves an oscillating frequency of 22.2 GHz while exhibiting an output power of 7.5 dBm with a phase noise of 110.1 dBc/Hz at 1-MHz offset. This work also demonstrates the highest operating frequency among previously published Si-based VCOs using micromachined structures. Index Terms—Micromachined inductors, voltage-controlled oscillators (VCOs).

I. INTRODUCTION

T

HE increasing demands on wireless data communication have motivated the development of radio frequency (RF) front-end circuits toward tens of gigahertz. Being a crucial component in wireless systems, voltage-controlled oscillators (VCOs) impose restrictions on both active and passive devices for the technology of choice. As the CMOS size advances to deep-submicron, CMOS fundamental oscillators at frequencies up to the millimeter-wave range were reported [1], [2]. However, it usually requires expensive process technology while delivering low output power. In order to implement low-cost oscillators for high-frequency applications, a cross-coupled push–push oscillator using a 0.25- m CMOS technology was proposed to achieve an output frequency twice that of the fundamental frequency with good fundamental rejection, of the transistors [3]–[5]. Though which is higher than the demonstrating high oscillating frequency and an acceptable output power, the phase noise performance of the push–push VCO is limited due to lack of high-Q on-chip inductors. In order to overcome the limited phase noise in a free running oscillator, some efforts have been made to alleviate the substrate loss by using bond-wires or integrated coils as the resonator. However, it is not suitable for system integration. Large suspended inductors on silicon were used to improve the performance of the CMOS RF amplifier [6]. Planar microwave and millimeter-wave lumped elements and coupled-line filters using Manuscript received July 1, 2005; revised August 2, 2005. This work was supported in part by the National Science Council of Taiwan, R.O.C. under Contracts NSC 93-2752-E-002-002-PAE, NSC 93-2219-E-002-024, and NSC 93-2213-E-002-033, and by TSMC and GCTC through the Chip Implementation Center (CIC), Taiwan. The authors are with the Department of Electrical Engineering and the Graduate Institute of Communication Engineering, National Taiwan University, Taipei, Taiwan, R.O.C. (e-mail: [email protected]). Digital Object Identifier 10.1109/LMWC.2005.860003

Fig. 1. Cross section of the CMOS layer structure before, during, and after micro-electro-mechanical system (MEMS) process.

micro-machining techniques to reduce the loss was reported in [7]. In this letter, we propose an oscillator implemented in a 0.35- m CMOS technology with a micromachined post-process. With the high-Q inductors introduced by the micromachined structure, the oscillator achieves an oscillating frequency of 22.2 GHz while exhibiting an output power of 7.5 dBm and a phase noise of 110.1 dBc/Hz at 1-MHz offset. II. MEMS PROCESS One primary limitation of the VCO phase noise results from -tank due to its inherently low the resonant inductor in the Q-factor. The degradation in Q-factor at high frequencies is mainly caused by the lossy substrate in CMOS technologies. In this work, a CMOS compatible post-process is presented for the implementation of the on-chip micromachined inductors without degrading the existing CMOS circuitry. The inductors are based on suspended metal structures consisting of four interconnection metal layers in a 0.35- m CMOS technology. As shown in Fig. 1, after the standard CMOS fabrication process, the Si wafer was patterned for bulk micromachining. The CMOS circuitry were covered with photo-resist. Silicon nitride and oxide layers were removed from the surface of the patterned openings by anisotropic reactive ion etching (RIE) with and O , followed by another isotropic etch gas mixture of process to remove the silicon substrate. With the undercut created by the isotropic etching, the Si substrate underneath the spiral coils was removed, leaving a suspended inductor structure on the wafer surface while the CMOS circuitry remained protected during the process [8].

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Fig. 2.

IEEE MICROWAVE AND WIRELESS COMPONENTS LETTERS, VOL. 15, NO. 12, DECEMBER 2005

Simulated Q-factor of the 0.13-nH inductors.

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Fig. 4. Chip photographs with the chip size 0.39 mm 0.9 mm of the 22.2-GHz VCOs with (a) micromachined inductors and (b) regular inductors on the silicon substrate.

Fig. 3. Schematic diagram of the 22.2-GHz micromachined oscillator.

III. CIRCUIT DESIGN Fig. 2 shows the simulated Q-factor of the inductor in comparison with a regular inductor using a commercial full-wave EM simulation tool, Sonnet [9]. The Q-factor is improved for the microachined inductor especially at frequencies above 15 GHz. According to the simulation results, a Q-factor of 17.5 is achieved at 22.2 GHz for the 0.13-nH inductor. In addition, the resonant frequency of the inductor also increases due to the reduced parasitic capacitance between the spiral coils, extending the application to higher frequencies. Fig. 3 shows the schematic diagram of the cross-coupled oscillator with micromachined inductors presented in this letter. A push-push oscillator design consists of two fundamental oscillators. Each one operates with a phase difference of 180 at half the desired output frequency. After the two signals combine at the output port, the fundamental signals cancel out each other, whereas the second harmonic signals are in phase and add up -tank and a constructively. Oscillator core is composed of a initiates oscillation at the cross-coupled pair. The negativefundamental frequency across the outputs of the cross-coupled pair along with the harmonic components. The output node is located in the middle of the inductor, behaving as a virtual ground for differential-mode and an open circuit for common-mode with respect to the cross-coupled pair. The out-of-phase fundamental components are cancelled while the in-phase harmonic components sum up at this node. As a result, an enhanced comresonance appears as ponent at the frequency twice of the the oscillator output. Note that a bias-tee is used at the output

Fig. 5. Measured phase noise at 1-MHz offset of the VCO with micromachined inductors.

Fig. 6. Measured phase noise at 1-MHz offset of the VCO with regular inductors.

node of the oscillator for the purposes of providing dc bias and extracting the push–push output. The 22.2-GHz VCO is designed using the micromachined inductors, while another one using the regular inductors on silicon substrate is also implemented for comparison. Fig. 4 shows the photographs of both VCOs with and without the micromachined inductors. Both of the same chip size of 0.39 mm 0.9 mm including the testing pads.


WANG et al.: 22-GHz PUSH-PUSH CMOS OSCILLATOR

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TABLE I COMPARISON WITH PREVIOUS PUBLISHED SI-BASED VCOs AROUND 20 GHZ WITH OUR WORKS

IV. MEASURED RESULTS

ACKNOWLEDGMENT

On-wafer probing was performed to characterize the performances of these oscillators. The measured output spectrum of the oscillator using the micromachined inductors at a supply voltage of 2.6 V is shown in Fig. 5. The oscillating frequency is 22.2 GHz with an output power of 7.5 dBm and the measured phase noise is 110.1 dBc/Hz at 1-MHz offset. The fundamental rejection is 18 dB. The relatively high dc power consumption of 143 mW is because a larger CMOS of 100- m gate width is used in the oscillator design for high gain. The performance of the oscillator can be evaluated by the figure-of-merit , carrier frequency , (FoM) including the phase noise , and total dc power consumption [10] offset frequency

The authors wish to thank M.-F. Lei and Z.-M. Tsai, National Taiwan University, for helpful discussions.

(1)

FoM

which results in an FoM of 175.5 dB for our oscillator with micromachined inductors. Fig. 6 exhibits measured phase noise at 1-MHz offset for the VCO with regular inductors on lossy Si substrate. At the same bias condition, the output power is 11.5 dBm with a phase noise of 103.8 dBc/Hz at 1-MHz offset. Table I summarizes the recently reported performance of Si-based VCOs around 20 GHz compared with our work. It is observed that a low phase noise VCO with better FoM is obtained by using the micromachined inductors and about 6-dB phase noise improvement is achieved compared with the free-running VCO with regular inductors. With the micromachined inductors, this CMOS VCO exhibits comparable FoM of those VCOs using SiGe HBTs [11]–[15]. V. CONCLUSION A 22-GHz micromachined push-push oscillator exhibits the low phase noise by adopting micromachined inductors. This chip was fabricated in a low-cost process while demonstrating the low phase noise. This chip is also the first 0.35- m CMOS oscillator with oscillating frequency over 20 GHz.

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