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Silicon germanium process heralds fast RF

Posted: 16 Sep 2005     Print Version  Bookmark and Share

Keywords:ibm  rf ic  silicon germanium  bicmos  gps 

Commodity RF ICs may ride to unheard-of frequencies on the strength of a new SiGe BiCMOS process from IBM Corp. Essentially doubling the performance that has been available in production BiCMOS, the 8HP and its lower-cost counterpart, the 8WL, are earmarked for applications that have yet to emerge into commercial reality, partly because of their high RF costs.

"There's a lot of attention paid today to automobile advanced cruise-control and collision-avoidance systems," said Bernard Meyerson, VP and chief technologist in IBM's systems and technology group. "These systems are based on a short-range radar operating at 24GHz or 77GHz. And there is increasing interest in personal area networks—wireless networks that can offer enormous bandwidth, but over a very limited area. There is a band allocated at 60GHz for these applications."

To become products, "these applications need a commercial production process with maximum performance well above 100GHz," Meyerson said. "That is what we are offering."

With an ft of 200GHz, the high-performance 8HP process will, for the first time, bring commodity-priced RF ICs to frequency bands far above 20GHz, IBM said. The low-cost 8WL version with roughly half the performance will be aimed at commercial wireless and GPS markets now served by more expensive high-end solutions.

"The role of SiGe in the industry has been to bring the economies of scale of silicon CMOS processing to higher-frequency applications," Meyerson said. "In the days when WLAN applications were running in the 2.5GHz to 2.7GHz range and required big GaAs front-ends, we introduced the first SiGe BiCMOS process. Intersil used it to dramatically lower the cost of 802.11 RF stages—at one point controlling about half the market." Now, he said, IBM is poised to work a similar transformation on next-generation RF.

Based on 130nm lithography and sharing both process technology and design intellectual property with IBM's 130nm CMOS logic and CMOS RF processes, the new SiGe BiCMOS technology will benefit from the experience in volume production and device modeling that IBM has accumulated at the 130nm node. But when it comes to signal paths that must function at tens of gigahertz or more, the process is a new and unique creature.

In SiGe BiCMOS, Meyerson said, "What drives performance, fundamentally, is your ability to control with incredible precision the epitaxial growth of the SiGe layer that forms the base of the bipolar transistor."

The new process, like its IBM predecessors, uses a conventional vertical bipolar transistor, with the base formed by a thin SiGe film grown between the doped-silicon emitter and collector bodies. "The active base region in this process is on the order of 100Å thick," Meyerson said. "In that thickness, it's precisely graded to give maximum carrier mobility. It really represents an extraordinary degree of control over epitaxial growth."

But the thin, graded base, which slashes carrier transit time and thus enables the great speed of the transistors, cannot do the job by itself. The design has to wage continuous war against parasitics, or all the potential speed would vanish before reaching the contacts of the transistor. Primarily, this means careful attention to overlay accuracy. "Misalignment increases parasitics," Meyerson said. "Precise alignment gives you low conductivity in the extrinsic base, which is vital to this level of performance."

Interconnect is also vital. At these frequencies, stripline techniques are necessary to get signals onto the chip and between the transistors. Without controlled impedance interconnect, losses in the wires would render most circuits unworkable. This means an unprecedented level of control, both in the formation of interconnect lines and in the accuracy of interconnect models.

Thick lines

IBM is using upper layers with thick wires in conjunction with the standard copper interconnect at the lower levels. Using multiple-contact via arrangements, signals are moved directly from the active devices to the thick wires, where predictable striplines can be formed. Thus, the variability issues that plague fine-pitched dual-damascene copper lines are avoided.

Passive components are also an issue at these frequencies. IBM is offering inductors and capacitors—the latter using a proprietary high-k dielectric material—with sufficient quality factors for most applications. "People designing narrow notch filters are probably still going to want to use off-chip passive components," Meyerson said. "But for most ordinary circuit passives, the on-chip components do very well."

Models, extraction tools and design tools have all been upgraded to handle the new frequency range, the company said. Oversights that were inconsequential at 2.5GHz could mean complete loss of a signal at 70GHz. To deliver an ASIC offering instead of a custom process, IBM had to fully support the 130nm SiGe recipe in this regard as well.

- Ron Wilson

EE Times




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