J.N. Burghartz

On the design of RF spiral inductors on silicon

This review of design principles for implementation of a spiral inductor in a silicon integrated circuit fabrication process summarizes prior art in this field. In addition, a fast and physics-based inductor model is exploited to put the results contributed by many different groups in various technologies and achieved over the past eight years into perspective. Inductors are compared not only by their maximum quality factors (Q/sub max/), but also by taking the frequency at Q/sub max/, the inductance value (L), the self-resonance frequency (f/sub SR/), and the coil area into account. It is further explained that the spiral coil structure on a lossy silicon substrate can operate in three different modes, depending at first order on the silicon doping concentration. Ranging from high to low substrate resistivity, inductor-mode, resonator-mode, and eddy-current regimes are defined by characteristic changes of Q/sub max/, L, and f/sub SR/. The advantages and disadvantages of patterned or blanket resistive ground shields between the inductor coil and substrate and the effect of a substrate contact on the inductor are also addressed in this paper. Exploring optimum inductor designs under various constraints leverages the speed of the model. Finally, in view of the continuously increasing operating frequencies in advancing to new generations of RF systems, the range of feasible inductance values for given quality factors are predicted on the basis of optimum technological features.

Substrate effects in monolithic RF transformers on silicon

The effect of substrate RF losses on the characteristics of silicon-based integrated transformers is studied experimentally by using a substrate transfer technique. The maximum available gain is used to evaluate the quality of transformers similarly to that of active devices. The silicon substrate has a pronounced effect on the quality factor and mutual resistive coupling factor of the primary and secondary coils, thereby degrading the maximum available gain of the transformer. A highly structured patterned ground shield is shown to improve the maximum available gain of a transformer at high frequencies, while at low frequencies, it has little effect on the maximum available gain and even degrades the quality factors of the transformer coils. It is shown that the low-frequency degradation of the coil quality factors relates to local eddy currents in the patterned metal shield.

Surface-passivated high-resistivity silicon substrates for RFICs

Surface passivation of high-resistivity silicon (HRS) by amorphous silicon thin-film deposition is demonstrated as a novel technique for establishing HRS as a microwave substrate. Metal-oxide-silicon (MOS) capacitor measurements are used to characterize the silicon surface properties. An increase of the quality factor (Q) of a 10-nH spiral inductor by 40% to Q=15 and a 6.5-dB lower attenuation of a coplanar waveguide (CPW) at 17 GHz indicate the beneficial effect of the surface passivation for radio frequency (RF) and microwave applications. Regarding CPW attenuation, a nonpassivated 3000-/spl Omega//spl middot/cm substrate is equivalent to a 70-/spl Omega//spl middot/cm passivated substrate. Surface-passivated HRS, having minimum losses, a high permittivity, and a high thermal conductivity, qualifies as a close-to-ideal radio frequency and microwave substrate.

Spray coating of photoresist for pattern transfer on high topography surfaces

In this paper, a new method of photoresist coating, direct spray coating, is studied. This method is especially suited to coat high topography surfaces for some special applications in microelectromechanical systems, radio frequency components and packaging. The most suitable photoresist type and coating process are found. The influence of several coating parameters on the thickness and uniformity of the photoresist layer is investigated. A model describing the dependence of the thickness on the major parameters is presented. Very promising results are obtained using spray coating for the fabrication of several three-dimensional structures.

Photoresist coating methods for the integration of novel 3-D RF microstructures

This paper presents three coating methods of photoresist on large three-dimensional (3-D) topography surfaces. Two special methods, spray and electrodeposition (ED) are introduced and investigated for the fabrication of 3-D microstructures and RF-MEMS devices. Characteristics of each method as well as its advantage and disadvantages are outlined. A comparison is made to point out the most suitable coating method in terms of complexity, performance and type of application. The potential of these coating methods is demonstrated through several applications such as fabrication of multilevel micromachined structures and RF MEMS devices.

RF potential of a 0.18-/spl mu/m CMOS logic device technology

The radio-frequency (RF) performance of a 0.18-/spl mu/m CMOS logic technology is assessed by evaluating the cutoff and maximum oscillation frequencies (f/sub T/ and f/sub max/), the minimum noise figure (F/sub min/) and associated power gain (G/sub a/), and the 1/f noise of the devices. Gate-biasing and channel-length and gate-finger-length adjustments are identified as means to optimize the RF performance without any technology process modifications. Changing to N/sub 2/O gate dielectrics is shown to greatly reduce the 1/f noise without sacrificing the AC performance. The power amplifier characteristics of CMOS at low power levels are also discussed.

Integrated tunable magnetic RF inductor

We demonstrate, for the first time to our knowledge, a passive, electrically tunable integrated radio frequency (RF) inductor based on a planar solenoid with a thin-film ferromagnetic(FM) (NiFe) core. Variation of inductance is achieved by leading an additional dc current through the same device, thereby changing the effective permeability of the FM core. Tuning ranges (relative variations in inductance) of 85%, 35%, and 20% are achieved at 0.1, 1, and 2 GHz, respectively, for inductances in the range of 1 to 150 nH.

Integrated solenoid inductors with patterned, sputter-deposited Cr/Fe/sub 10/Co/sub 90//Cr ferromagnetic cores

Ferromagnetic (FM) films suitable for implementation in between interconnect layers of a standard CMOS fabrication process are demonstrated to yield considerable size reduction of monolithic radio frequency (RF) inductors, leading to lower cost. The deposition of a FM Cr(5 nm)/Fe/sub 10/Co/sub 90/(500 nm)/Cr(15 nm) stack is performed by magnetron sputtering at room temperature under a dc magnetic field of /spl sim/ 10 mT along the magnetic easy-axis. A lift-off technique, using a four-layer shadow mask, is used for pattern transfer to the magnetic stack to circumvent apparent difficulties in the patterning of FM films. A series of solenoid-type inductors with FM cores are demonstrated and compared to control devices with air cores. A more than eight-fold enhancement of the inductance and a seven-fold improvement of the quality factor are achieved.

Surface-passivated high-resistivity silicon as a true microwave substrate

This paper addresses the properties of a surface-passivated (enhanced) high-resistivity silicon (HRS) substrate for use in monolithic microwave technology. The detrimental effects of conductive surface channels and their variations across the wafer related to the local oxide and silicon/silicon-dioxide interface quality are eliminated through the formation of a thin amorphous layer at the wafer surface. Without passivation, it is found that the surface channels greatly degrade the quality of passive components in HRS by masking the excellent properties of the bulk HRS substrate and by causing a spread in parameters and peak values across the wafer. Moreover, it is seen that the surface passivation leads to excellent agreement of the characteristics of fabricated components and circuits with those predicted by electromagnetic (EM) simulation based on the bulk HRS properties. This is experimentally verified for lumped (inductors and transformers) and distributed (coplanar waveguide, Marchand balun) passive microwave components, as well as for a traveling-wave amplifier, through which also the integration of transistors on HRS and the overall parameter control at circuit level are demonstrated. The results in this paper indicate the economically important possibility to transfer microwave circuit designs based on EM simulations directly to the HRS fabrication process, thus avoiding costly redesigns.

Ferromagnetic RF inductors and transformers for standard CMOS/BiCMOS

Integrated solenoidal radio-frequency (RF) inductors and transformers with a ferromagnetic core are demonstrated by using a fully IC-compatible fabrication process. For a 500-nm-thick Ni/sub 80/Fe/sub 20/ film, a cut-off frequency of over 7.5 GHz for a 2.5-nH inductor is realized. Optimum inductor design based on tailored inductance, quality factor, and cut-off frequency is discussed. RF transformers realized have a coupling factor of /spl sim/70% up to 200 MHz.