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.
As SiGe pushes silicon deeper into the RF domain, the technology is extending into a broad range of new products as an alternative to GaAs devices. Encouraged by the progress, Tektronix recently adopted SiGe instead of expensive GaAs technology in the design of a digital phosphor oscilloscope (DPO) with a 4-GHz bandwidth. This was in order to keep pace with the higher data rates encountered in next-generation instrument designs. With capture rates of 500,000 waveforms/s, the SiGe-enabled DPO provides accurate measurements at significantly increased speeds over traditional silicon at similar power levels, asserts Tektronix.
Furthermore, the high level of integration afforded by the SiGe technology gives the new-generation scopes excellent signal integrity with a substantially lower noise floor compared to previous amplifier designs, states the instrument maker. Tektronix is planning to expand this capability to other product lines like logic analyzers, data generators, and probes, as well as hand-held instruments.
Additionally, IBM's user list keeps growing. To strengthen its position even more in the 10- and 40-Gbit optical networking space, IBM has signed new deals with Multilink Technology Corp. and Vitesse Semiconductor Corp. Together, IBM and Multilink are planning to design and develop components for high-speed transport-layer 10- and 40-Gbit synchronous optical network (SONET), synchronous digital hierarchy (SDH), and Ethernet domains. Actually, the two are working to offer complete solutions, including reference designs, for these emerging high-speed applications.
Interestingly, IBM also has taken a minority stake in Multilink, a supplier of telecom and data-communications ICs. Under a separate agreement, Vitesse will gain access to IBM's SiGe process for developing novel solutions for optical networks. A strong supporter of GaAs, Vitesse now sees SiGe bipolars as a complement to its in-house arsenal of compound-semiconductor technologies.
Along these lines, Ray Milano, vice president of optoelectronic products at Vitesse, says, "IBM's SiGe biCMOS process provides the flexibility and integration capabilities we need to develop ICs that meet the rising bandwidth and speed requirements of next-generation optical gear." Employing IBM's SiGe biCMOS, Vitesse has been developing a couple of ICs during the last six months. These include a parallel transmitter array for driving VCSELs and a TIA for a receiver front end. "To meet the on-chip logic requirements and power constraints of these ICs, SiGe was the best choice," states Milano. "Plus, it offers high gain per stage to minimize the number of stages, thereby minimizing group delay, which is critical at these frequencies and applications. Obviously, these circuits cannot afford too many gain stages, and SiGe becomes a natural fit."
Meanwhile, Temic Semiconductors continues to expand its SiGe product portfolio with more integration on-chip. Now the supplier is building on its experience of designing low-noise amplifiers (LNAs) and power amplifiers to make ready RF front ends for wireless communications. Using a state-of-the-art 0.35-µm SiGe biCMOS process, Temic has launched a monolithic RF front end, which includes a low-noise amplifier (LNA), a power amplifier, and a transmit/receive switch driver for the radio part of the home-networking solution.
According to Temic, the RF front-end T7024 is compliant with Bluetooth specifications, delivering an output power of +23 dBm at 3 V and a gain of 25 dB (Fig. 2). The SiGe transistors keep current consumption to a low 170 mA. The typical noise figure for the chip is 2.0 dB.
Concurrently, the manufacturer continues to streamline its standalone LNAs and power amplifiers. Two recent additions were dual-gain LNAs for 925- to 960-MHz GSM phones, and 1.8- to 2.0-GHz DCS/PCS handsets. By implementing a switchable gain stage between amplifier sections of these LNAs, Temic designers have obtained a low-noise figure (2.2 dB) in a high-gain mode with low intermodulation distortion. A −40-dB reverse isolation ensures that no power is emitted between the mixer and the antenna in direct-conversion applications. This also guarantees a high-dynamic range for maximum reception at both very small and very large signal levels. Aside from reverse isolation, the LNA's ability to precisely control gain determines its sensitivity to signals. The nominal gain is 19 dB with a ±1.0-dB variation.
Analog Devices, in reality, isn't new to SiGe technology. Initially, about five years ago, the company investigated IBM's process for building high-speed 12-bit DACs. Because its own proprietary high-speed complementary bipolar, labeled XFCB, came up to speed to serve those needs, ADI had to put SiGe on the back burner. But, rapid improvements in the SiGe process in the last few years have changed that scenario. The dynamics are now in favor of SiGe and ADI is back in the game. It's actively pursuing the development of SiGe HBTs for incorporation into its XFCB family, as well as replacing the conventional silicon bipolars with SiGe HBTs in its biCMOS process. In fact, after having experimented with 0.6- to 0.8-µm feature sizes, ADI is in the process of shrinking the transistor size to become compatible with its 0.35-µm CMOS process. Target fT for npn HBTs is 50 GHz.
According to Brad Scharf, ADI's division fellow and manager of process development, "The SiGe process will support both 3-V and 5-V HBTs on the same chip." He adds that "this process will scale with CMOS." Several RF chips aimed at front-end functions for wireless and networking systems, pin drivers and timing electronics for ATE equipment, and amplifiers for DSL and cable modems are in the works. Some of these ICs will be in sampling mode by the year's end.
Progress in the SiGe field and its suitability in the wireless realm has attracted many more players. Some key backers include Analog Devices Inc., Conexant Systems Inc., Infineon Technologies, Maxim Integrated Products, Motorola Inc., Philips Semiconductor, STMicroelectronics, and many more. Several of these newcomers are in the process of ramping up their production facilities and releasing parts based on their proprietary SiGe schemes.
Motorola, for instance, has successfully merged a SiGe:carbon process module, developed in cooperation with High Performance Microelectronics of Frankfurt, Germany, into a mainstream RF biCMOS technology. By doing this, the developers ensured that the digital elements are parametrically matched to the merged process, and HBT performance is optimized. Only a few additional masking layers are required to add SiGe:C HBTs to the standard 0.35-µm CMOS process. Furthermore, the designers have refined the fabrication flow for a smooth transition to 0.18-µm design rules as the process is scaled for next-generation ICs.
Motorola's SiGe:C-based HBTs boast an fT of 50 GHz and fMAX of 70 GHz at a 2-V VCE. Additionally, the current drawn is half that of traditional SiGe bipolars, according to Motorola. Armed with SiGe:C biCMOS, Motorola designers are melding high-performance RF functions with dense 0.35-µm CMOS on the same RF substrate.
As for integration, the supplier's strategy is to develop a library of building blocks, such as LNAs, mixers, downconverters, and other similar functions. Next, Motorola will combine these functions into single-chip subsystem solutions targeting custom designs. Standard products will be derived from this effort as well, according to Olivier Lauvray, technology manager for Motorola's RF/IF division.
Multiband, multimode RF ICs using SiGe:C-based biCMOS devices are in development. One such dual-band CDMA RF front-end packs an LNA and a mixer along with VCOs and copper transformers on the same chip (Fig. 3). Samples of the RF front end are expected late in the third quarter, with production planned for the fourth quarter.
Also joining this manufacturing fray, Conexant Systems is attempting to catch up rapidly. The company has readied its first-generation SiGe technology on the heels of putting silicon biCMOS in place. Conexant intends to exploit both processes for a range of products.
The company has begun manufacturing new SiGe-based communications chips for low-power wireless and high-speed networking applications. The company's SiGe devices are being fabricated in the supplier's high-volume wafer fabrication facility in Newport Beach, Calif.
"We have chosen 0.35 µm as our first node because it's easier to deal with. And, it's adequate for the level of integration required," states Paul Kempf, Conexant's director of platform technologies. According to Kempf, the company has augmented its proven 0.35-µm biCMOS with SiGe epitaxy and deep-trench isolation techniques. "Unlike others," he adds, "the availability of advanced lithography equipment has allowed the developers to implement a nonself-aligned emitter to keep the complexity of the process low. As a result," he continues, "the mask count is similar to that of conventional biCMOS, even though it incorporates deep-trench isolation and a SiGe base epitaxial layer" (Fig. 4).
Kempf further comments that "aside from incorporating a variety of passive components, including inductors, for higher integration, the Conexant process possesses the ability to produce both 3.3- and 5-V bipolar transistors. In addition, the process also implements a good varactor, enabling designers to integrate a VCO on-chip."
Capable of delivering an fT of 45 GHz at a VCE of 1 V, Conexant SiGe transistors draw much lower current than some of the competing technologies. "Our aim is to provide good performance at a low-current density of 40 µA/µm2. Efforts are in progress to boost that figure to 60 GHz and beyond in the next-generation technology," adds Kempf.
Getting Smaller Still
Speaking of the future, migration toward 0.18-µm CMOS is in the works as well. With that in mind, Conexant is currently refining the process and tweaking the deep-trench to be compatible with the shallow-trench of the 0.18-µm design rules. Leveraging its 0.35-µm SiGe-based biCMOS technology, Conexant has released two ICs for use in 2G- and 3G-CDMA handsets. One, the CX74002, is a dual-band transmit IC with a built-in I-Q interface, dual PLLs, and an image-reject mixer.
The other, the CX74004, is a tri-mode, dual-band LNA downconverter. Even though the level of integration is higher, the parts are designed for low current consumption at 3.3 V. Housed in LGA packages, both SiGe parts are presently sampling with production slated for the year's end. Later in the year, Conexant also expects to bring online the higher-frequency version of the SiGe process. The company plans to produce chips compliant with OC-192 SONET specifications for use in fiber-optic networking gear.
Driven by SiGe HBT's higher gain and low noise, aside from higher fT and fMAX, Maxim Integrated Products has developed in-house a proprietary process. Maxim has been tapping its home-grown SiGe technology for over a year. During this period, the company has added several new members to its SiGe family.
The latest to join is a SiGe bipolar array, labeled QuickChip 11. In essence, QuickChip is an array of uncommitted semiconductor components held in the inventory, awaiting metal interconnects to implement a custom IC. It's Maxim's low-cost, rapid response ASIC service, designed to reduce layout and wafer-fabrication time. Based on this approach, QuickChip 11 offers 2684 npn transistors of various sizes, 636 lateral pnp bipolars for biasing, 288 Schottky diodes for level shifting, 6668 polysilicon resistors, 76 stacked 2-pF capacitors, and 48 bond pads in a 2.4-mm2 die (Fig. 5).
Over and above, this new bipolar array also features the ability to place custom trimmable low-temperature-coefficient resistors and high-value metal-insulator-metal (MIM) capacitors on the same die. Arranged in six different repeated tiles, the QuickChip 11 is optimized for high-performance analog and digital circuits. It's supported by a comprehensive set of design-verification and layout tools for Unix and PC platforms.
Furthermore, Maxim also has developed a dual-band LNA plus mixer for cellular phones. The MAX2323 is an improvement over its predecessor. Enhancements include the addition of a third gain stage for a better switchover-hysteresis margin, an increased input third-order intercept point (IP3), and a smaller package. Additionally, the mixer's noise figure has been reduced for better sensitivity.
Likewise, using 0.25-µm SiGe biCMOS with an fT of 75 GHz, Infineon Technologies has unwrapped a five-piece SiGe chip set for SONET/SDH applications. After demonstrating the viability of fabricating SiGe HBTs last year, STMicroelectronics is gearing up this year for production. Similarly, Philips Semiconductor is preparing a 0.18-µm biCMOS process infused with SiGe npn bipolar devices. Philips is hopeful of unveiling parts later in the year.
The quest to improve performance without a cost penalty has motivated the researchers of Lucent Technologies' Bell Labs to develop a novel super self-graded SiGe base transistor. It employs high-energy implantation rather than epitaxial growth to form the subcollector region (Fig. 6). With only four additional lithography layers over the 0.25-µm CMOS process, Bell Labs' scientists have demonstrated an npn HBT with an fT of 52 GHz and an fMAX of 70 GHz. The transistor breakdown voltage is 4 V, while the forward Early voltage is 40 V.
The researchers have demonstrated the viability of the technology by building a 4:1 multiplexer and 1:4 demultiplexer for use in 10-Gbit/s fiber/data links. The results were presented at the last IEDM.
The craving to keep pushing the performance envelope has motivated researchers at Hitachi's Central Research Laboratory to squeeze more from the SiGe HBT. With the goal of attaining an fT greater than 100 GHz and less than a 10-ps gate delay, Hitachi researchers have constructed a self-aligned selective epitaxial-growth SiGe HBT with shallow trench and dual-deep trench isolations. Fully compatible with a standard 0.2-µm biCMOS process, this architecture yielded a maximum oscillation frequency of 107 GHz and an ECL gate delay time of 6.7 ps at a switching current of 1.3 mA.
In reality, this structure is designed for low current density of about 3.5 mA/µm2. The researchers attribute the ultra-low power and ultra-fast response to low collector capacitance and high fT/fMAX of the fully self-aligned SiGe HBT structure, low parasitic resistance of the electrodes, and low substrate capacitance enabled by shallow-trench and dual-deep-trench isolations.
As SiGe technology makes major strides to pervade the 900-MHz to 2.4-GHz communications space, it's gaining support from wireless and fiber-optic systems designers around the globe. The technology is poised for growth and possesses the ability to spawn many new solutions for a new wave of products.
Nevertheless, there are still many challenges that confront SiGe bipolar and associated biCMOS processes from system-on-a-chip (SoC) standpoints. The ability to scale smoothly with ultra-deep-submicron CMOS, as silicon enters the nanometer region, as well as the flexibility to add microprocessors/DSPs and memories, will chart its future course.
|Suppliers Mentioned In This Report|
Analog Devices Inc.
Conexant Systems Inc.
Vitesse Semiconductor Corp.