[Technology Report]
Silicon-Germanium HBTs Merge With Mainstream CMOS Process
The resulting SiGe biCMOS process melds RF and analog functions with deep-submicron CMOS logic on the same silicon substrate.
Only a few years ago, there was tremendous uncertainty about the commercial feasibility of transforming homojunction silicon bipolar transistors into silicon-germanium (SiGe) heterojunction bipolar transistors (HBTs), for higher-switching frequencies. But, the continued refinements of doping silicon with germanium over the last few years has paid off. Today, many hurdles have been overcome and production issues alleviated to smoothly achieve this metamorphosis.
Furthermore, enhancements in the performance of SiGe-based HBTs are being coupled with the mainstream CMOS process to push biCMOS technology deeper into the RF domain. This will accomplish system-level integration of RF, analog, and digital functions on the same die. In short, SiGe-based devices are poised for growth, posing an even greater threat to gallium-arsenide (GaAs) devices on the latter's own turf.
As a result, major proponents like IBM Microelectronics and Temic Semiconductors, a wholly owned subsidiary of Atmel Corp., are in high-volume production of SiGe HBTs. In fact, IBM has moved its third-generation SiGe technology into the production phase, which includes 0.4- and 0.25-µm biCMOS capabilities. Plus, its SiGe roadmap speaks of the company's commitment to continue improving the RF performance of the HBTs, as well as its integrability with the state-of-the-art CMOS technology. Moving toward that goal, IBM developers will be implementing copper interconnects in 0.18-µm SiGe biCMOS by the year's end (see the table).
To meet the needs of emerging high-frequency applications, where a transition frequency (fT) of nearly 100 GHz is expected, IBM developers are extending SiGe HBTs' reach to 0.18-µm design rules. By scaling the HBT structure in both lateral and vertical directions, IBM designers have pushed the fT envelope to 90 GHz. This scaling is made possible by exploiting the same advanced lithography tools employed in the base CMOS technology, claims IBM. Consequently, the researchers have demonstrated the integrability of scaled SiGe HBTs with IBM's 0.18-µm, 1.8/3.3-V copper metallization CMOS process with little effect on CMOS device properties and design rules.
While the integration of the HBT with CMOS circuits utilizes the same methodology as the previous generation, the critical aspect of the next-generation IBM process is the post-base formation. In this base-after-gate integration approach, the high-temperature CMOS processing is done prior to the HBT base formation. Plus, the base epitaxy is grown in low-temperature processing.
There are several benefits of the smaller device structure. Among them, reduced parasitics and decreased base resistance are crucial to achieving a substantially higher fT and an improved noise figure. According to a paper written by IBM researchers for last year's International Electron Devices Meeting (IEDM), a minimum noise figure of 0.4 dB at 2 GHz has been obtained for a 0.18-µm-wide emitter stripe. In addition, the use of copper as a first metal layer provides substantial flexibility for wiring into high-current-density HBTs with less concern for electromigration, notes the paper.
Furthermore, the IBM paper discloses that copper provides low resistance between metal layers, contributing to the performance of RF circuits. Because the process incorporates a thick final-metal aluminum layer, it enables the integration of high-Q inductors and transmission lines, as well as the integration of passive components on the same substrate. What results is an unparalleled combination of high-speed mixed-signal and RF functions on the same die.
Incidentally, IBM scientists have also developed a simulation technique that profiles a SiGe device for a given geometry and doping. Through the application of such a model, IC designers can minimize the noise figure without sacrificing gain, linearity, frequency response, or the stability of the SiGe's strained layer. A SiGe HBT has the ability to switch at very high speeds, much beyond the speeds necessary for a lot of the bulk wireless space operating up to 2.4 GHz. Therefore, the excess speed can be traded for improvement in power. So, by reducing the operating current of the transistor, a designer can trade excess speed for significantly reduced power consumption (Fig. 1).
In addition to internal developments, IBM has also inked many partnerships to co-develop SiGe chips as well as offer foundry services. Consequently, more than a dozen semiconductor suppliers and systems houses around the world are tapping IBM's SiGe manufacturing services to address a broad range of commercial, consumer, and instrumentation applications. They're also realizing new performance benchmarks and standards, while enabling novel solutions for a new wave of products.
For instance, harnessing IBM's SiGe manufacturing services, Applied Micro Circuits Corp. (AMCC) has revealed an unprecedented 34- by 34-differential crosspoint switch with over 100-Gbit/s switching capacity. In addition, it has unwrapped a 2.5-Gbit/s quad transimpedance amplifier (TIA) as well. Called the S7025, it has an integrated limiting amplifier and loss-of-signal detection circuitry. In fact, this TIA is part of a chip set aimed at parallel optical links of up to 300 meters. The other member in this set is the 2.5-Gbit/s quad vertical-cavity surface-emitting laser (VCSEL) driver, the S7022. In conjunction with the S3457 transceiver, the chip set provides a 10-Gbit/s bi-directional very short-reach OC-192 link.
"We have just begun to exploit the capabilities of SiGe for high-speed communication IC designs," notes Greg Winner, AMCC's vice president of engineering. He adds that "it provides ultra-high speed coupled with superior jitter performance, low power, and low price." Featuring data rates of 3.2 Gbits/s/channel and 2-ps typical root-mean-square (RMS) jitter accumulation, the 34- by 34-crosspoint solution is aimed at dense-wavelength-division-multiplexing (DWDM) applications in optical networks.
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