Darrin J. Young
Wireless Microsystems Lab

Monolithic Micromachined RF Low Phase Noise VCOs

Darrin J. Young

Monolithic communication transceivers are highly desirable for wireless communication applications. However, current radio designs depend on off-chip components to implement key building blocks such as the low phase noise RF voltage-controlled oscillators (VCOs). In most systems the oscillators employ discrete inductors and varactor diodes for frequency tuning. These off-chip devices rely on processing and materials that differ substantially from standard IC fabrication and are consequently not suitable for monolithic integration, thus increasing cost, size, and packaging complexity.

The goal of this project is to build a complete monolithic low phase noise VCO using on-chip IC-compatible, high-Q variable capacitor and inductor. The variable capacitor has been accomplished through building a surface-micromachined all-aluminum microstructure. The fabrication procedure is fully compatible with a standard IC integration. The research results were presented at the 1996 IEEE Sensor and Actuator Workshop . Figure 1 shows an SEM of the top-view of a fabricated capacitor with a nominal capacitance value of 200 fF. The device consists of a thin aluminum plate (200 um x 200 um) suspended in air nominally 1.5um above a bottom aluminum layer by four mechanical springs. Figure 2 shows the suspended plate and beam. A DC bias applied across the capacitor causes an electrostatic pull-down force and consequent reduction of the air gap, resulting in an increase in capacitance. Aluminum is selected for structural layer to ensure a low sheet resistance and a high quality factor (Q) of the capacitor even at high frequencies, a key parameter to meet low phase noise requirements of high-performance communication systems, such as cellular telephony. Figure 3 shows four such devices connected in parallel achieve a capacitance value of 2.04 pF, a typical value for RF VCOs. The capacitance increases to 2.35 pF under a 3 V tuning voltage. The device has a high Q value of 62 measured at 1 GHz.

A high-Q inductor is another critical key component to achieve a low phase noise VCO. Recently on-chip three-dimensional (3-D) high-Q coil inductors were designed, fabricated, and tested. The fabrication process in fully compatible with standard IC integration. The research results were presented at the 1997 IEEE International Electron Devices Meeting . Figure 4 shows an SEM of a fabricated 3-D one-turn coil inductor on a silicon substrate, achieving an inductance of 4.8 nH and a record Q of 30 at 1 GHz. The device consists of 5 um-thick copper traces electroplated around an insulating core with a 650 um by 500 um cross-section. Compared to spiral inductors, this geometry minimizes the coil area which is in close proximity to the substrate and hence the eddy current loss, resulting in a maximized Q factor and self-resonant frequency of the device. Copper is selected as the interconnect metal because of its low resistance, critical for achieving a high Q. A 3-D four-turn inductor, shown in Figure 5 , obtains 14 nH inductance and a Q of 16 at 1 GHz.

A two-turn 3-D coil inductor, micromachined capacitors, and separately fabricated CMOS Colpitt's oscillator electronics die are attached to a test board and wire bonded to form a VCO. This hybrid approach is taken to reduce the complexity of the prototype oscillator. Figure 6 shows the oscillator output power spectrum at 863 MHz. The VCO can be tuned from 855 MHz to 863 MHz under 3V. limited by the test board parasitic capacitance. Figure 7 shows the measured phase noise plot indicating that the VCO achieves a phase noise of -136 dBc/Hz at 3 MHz offset frequency. This level of performance is suitable for most wireless communication applications, in particular GSM cellular telephony, and has not been achieved by VCOs relying on conventional on-chip spiral inductors and silicon junction variable capacitors. The research results were presented at the 1998 IEEE Sensor and Actuator Workshop . Because all the key components are fabricated on silicon wafers, they are amenable to integration on a same substrate to achieve a complete monolithic low phase noise RF VCO, which is planned for the next step.

The on-chip high-Q passive components demonstrated here are not only critical for achieving miniaturized high-performance RF VCOs, but also crucial for implementing LC-based RF front-end bandpass filters, low-loss matching networks, RF low-noise amplifiers (LNAs), and high-efficiency power amplifiers. These key building blocks all need to be monolithically integrated in order to ultimately miniaturize the radio transceivers for wireless applications.