Pricey Processes For Low Power

It’s still technically feasible to create chips at advanced processes, but economic constraints are narrowing the field.

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By Pallab Chatterjee
Recently Samsung gave an update on the status and availability of its advanced 32/28nm process technology for use in foundry. The process is targeted for shipping designs to customers at the end of this year, with a road map that continues through the 22/20nm nodes and down to 15nm.

What was particularly interesting were several key innovations that have made this all possible, as well as the company’s statement that the real driver is reduced power.

The new processes, co-developed with IBM, follow the large commercial success of the Intel achievement of using a Hafnium “Hi-K” metal gate process. Although this terminology has been around for a few years and is the dominant technology in the microprocessor marketplace, there has been some “uncertainty” in the design community about what it actually buys the designer. The Hi-K gate technology is a process development that directly addresses the leakage current problem that arose in CMOS technology at the 90nm node and has persisted through the 45nm node. The scaling on process technology using Moore’s law is a three-axis scaling—x and y for the length and width of the transistor used to make the basic devices, and also z or the vertical dimension. Z is the thickness of the gate dielectric, which controls the intrinsic speed and performance of the device by setting the difference between “on” and “off.”

Since the late 1960’s the scaling of all three axes has taken place concurrently—until the 90nm node, that is. At 90nm the complexities of lithographic processing, planarization, materials used for interconnect, isolation between devices and reduction in application power supply were moved up from third- to fourth-order issues to become the dominant drivers. This made the leakage current and capacitance issues with the z-direction scaling the secondary challenge. This focus on the other processing issues caused the gate scaling to stall, and not continue proportionately with the x and y scaling, resulting in leakage, multi-power islands, high electric fields, and high-stress devices and designs that have dominated the past few years.

The lithography solution is staying optical with multiple patterning solutions through the 22/20nm node. The planarization, interconnect and device stacking for “multi-die” technologies are progressing to address the function vs. density vs. space requirements going forward, which allowed time to develop the new materials needed to make the gate dielectric (replacement of standard SiO2 with an Hf based material) and re-start the z-dimension scaling. At the 32/28nm node, the reduced leakage and increased device performance (difference between “on” and “off” states) brings a new level of design capability.

Results using the process in foundry-type circuits (embedded processors with memory, custom logic, and standard commercial interface connectivity) are showing as much as a 35% power reduction for the same operation specification as existing circuits. This power reduction comes from both the ability to drop the operating supply voltage for the same performance specification and from an overall reduction in leakage/standby state power for “idle” modes in a design.

The new process technology, now starting to become available from multiple suppliers, does bring an opportunity to create a new generation of mobile appliances. There is a significant challenge to the design community to address these benefits as a mainstream technology solution. The cost of entry into the design game at these nodes is very high. A typical 32/28nm SoC is probably going to contain more than 500 million devices, including embedded memory, and will likely have a very high pin count. This will require a big design team to architect, design, assemble, and test, not counting the very aggressive 20-plus man-years of IC design (5M devices/man year for the flow X 20 people = 100M devices + 400M in third party embedded memory), and application software development.

These design costs are on top of the fab costs, which are targeted at more than $4M for masks, plus the wafer fab, package and test. And it is looking like the big boys at the $30 million-minimum per design are the only ones who will be left at the table for real “low power” process game.