A Comparison of RFIC Fabrication Technologies

By Gary Breed Editorial Director

ontinued growth cheap and the volume of work in the digital This tutorial provides a brief in wireless, optical realm has already brought down the cost of overview of the capabilities Cand other high- fabrication to rock bottom. and limitiations of major speed, high-frequency However, the challenges for making high- integrated circuit technolo- products has created a frequency ICs in silicon have been enormous. gies used for RF and other demand for integrated When first explored for RFICs, CMOS was the high frequency devices circuits that are opti- worst available process for noise performance, mized for performance, passive component integration, electrostatic cost and practical development complexity. discharge (ESD), and device modeling. These radio-frequency ICs (RFICs) represent Concentrated research efforts have dramati- a wide range of requirements that continues cally reduced the performance gap between to be met with an equally wide range of solu- CMOS and other popular processes such as tions, from low-cost ubiquitous CMOS to silicon BiCMOS or its SiGe variant. advanced III-V materials. CMOS has performance advantages in In this tutorial, we’ll take a look at the addition to the obvious cost advantage. First, major IC fabrication processes used for RFICs. it’s digital heritage has made new RF CMOS Although there are variations among the var- processes a logical choice (pun intended) for ious facilities, there are general performance high-speed digital circuitry, including impor- ranges that each process fits into. Hopefully, tant RF building blocks such as frequency this note will provide an introductory-level synthesizers and oscillators, switches and understanding of the choices an engineer baseband processors. Driven by the computing must make when choosing the right process to industry and Moore’s Law, geometries of 0.18 manufacture the device required for the end µm allow higher frequency operation simply product being developed. through reduction in physical size. These Fortunately for that engineer, the demand small features also reduce power consump- and the range of performance required has tion, but at a cost in noise performance and resulted in some innovative advancements in signal handling capability. IC design and fabrication. We will try to make CMOS is promising for future RFICs with note of as many specific features as possible as even more ambitious cost/performance we review the various processes. demands. Because this process has improved by such a dramatic amount in a relatively RFICs in CMOS short time, it has many supporters in the Complementary metal-oxide semiconduc- RFIC development community. tor (CMOS) processes are the cheapest, offer- ing high yields, large wafer size, and many SiGe BiCMOS and HBT fabs with years of experience making ICs of all This process was chosen next, because of types, from millions of computer memory its balance among performance, cost and com- chips to general-purpose analog and digital plexity. Making SiGe RFICs in the fab is only building block devices. The raw material is a small degree more difficult than ordinary

52 High Frequency Electronics High Frequency Design RFIC TECHNOLOGIES silicon CMOS, bipolar and BiCMOS processes. The initial GaAs is often the preferred process for higher fre- difficulty was in the development of consistent doping quencies, lowest noise, and highest power, compared to profiles and accurate design and process modeling tools. the other available processes. Various other III-V semi- These kept the cost of SiGe high for a time. conductor processes allow even higher frequency opera- Current SiGe heterojunction bipolar transistor (HBT) tion and other performance advantages. technology offers FT competitive with GaAs, the first real microwave IC technology, although GaAs still offers bet- Other Processes ter integration of passives and better power-handling Classic silicon bipolar and BiCMOS are still very pop- capability. When high frequency operation with very good ular, given their maturity and low cost. Below 1 GHz, sil- noise performance, mixed-signal capability, and low icon BiCMOS is probably the most-used process for IF cost/high volume manufacturing, SiGe is often the process signal processing, high-speed operational amplifiers and of choice. data conversion products. Where standard digital func- tionality is less important, low cost silicon bipolar tech- GaAs and other III-V Semiconductors nology is used more often than many industry observers For overall performance in microwave circuits, com- might expect. pound semiconductors have significant performance Silicon-on-insulator (SOI, also silicon-on-glass or sili- advantages. The design of the circuit and its implementa- con-on-sapphire) is an interesting CMOS or BiCMOS pro- tion on the IC substrate are simpler than with the silicon- cess, where the traditional silicon substrate is thinned based processes. As noted above, power handling and on- and mounted to a supporting insulating layer. This con- chip passives, particularly inductors, is superior to the figuration results in very high isolation between circuit other processes. An early drawback was in modeling. elements and reduces noise. Both of these characteristics Traditional IC processes relied on time-domain (SPICE) are important for RF circuits, and SOI is used for low analysis of the circuit to verify its performance in final noise phase-locked loops and high-isolation analog and form, fabricated on a semiconductor substrate. With RF switches. microwave circuits used on GaAs, the distributed elements and passive components could not be accurately modeled Design Tools in the time domain. As a result, GaAs evolved along a Finally, none of the advances in any IC process would modeling path that was distinct from silicon processes. be possible without a parallel advance in software for the This caused an “either/or” approach within entire compa- design, layout, simulation and verification of the device. nies, and gave rise to GaAs specialized tools that had no Foremost among the advances has been a steady merging use in the silicon realm. Only recently have the barriers of classic RF/microwave circuit design tools and IC between the process types been removed, as the simula- design, layout and verification tools. Although this pro- tion and modeling tools became more comprehensive. cess is not complete, nearly all of the former barriers have GaAs has improved with time, as have all the other been removed—the circuit design software can share data available processes. Costs have been reduced and the easily with the IC design software. recent increase in the use of pHEMT technology has pro- Any engineer beginning the process of design that will vided a performance enhancement, restoring the high-fre- result in an RFIC solution must understand both the fab- quency advantages that seemed to be eroding as SiGe was rication process and the modeling tools that will be used developing. to design the circuit and create its IC implementation.