EUV Focus Shifts To Affordability

By David Lammers
Over the past year, key technologists in the semiconductor industry have come around to believing that EUV lithography will be available for critical mask layers in the next three to five years. What is still up for debate is whether EUV will be cost-effective for low-power consumer SoCs. To penetrate that cost-sensitive market, EUV must overcoming hurdles presented by masks, damage to the collector optics, power consumption, and other costs of operation.

Memory makers, particularly the DRAM vendors with less-regular 2D structures than the more one-dimensional NAND devices, are fully on board the EUV bandwagon. However, several SoC makers such as Texas Instruments are on the fence. At the Sematech Litho Forum, TI’s Jim Blatchford, manager of lithography development and process simulation, argued that double and triple patterning with 193i scanners will better meet TI’s cost and performance objectives over the next five years.

TI seeks a 30% to 40% reduction in the node-to-node cost per die. For the 60nm routed pitch layers, corresponding to the 22/20 nm node, double patterning “is the only economically feasible solution,” Blatchford said in a detailed presentation on “lithonomics.” For the 14nm node, triple patterning will be required for the gate, contact, and metal 1 layers. “Double and triple patterning will be cost effective for die with a routed pitch of about 40nm,” he said.

“Triple patterning at the 44 nm pitch provides a 25% better cost reduction,” Blatchford said, adding that TI expects to use “aggressive” double patterning to meet its cost objectives at the 20nm node and “multi-patterning” at the 14nm node.

“The choice of lithography is an economic decision,” Blatchford said at the Sematech meeting. On the technical side, TI requires an overlay specification of 6nm to 10nm for triple patterning to work out. “We need another click in the overlay improvement from the scanner manufacturers,” Blatchford said.

Samsung expects EUV lithography to be less than half as expensive as double patterning. (Source: Samsung at the 2010 Sematech Litho Forum)

Most logic companies are cheering on the EUV technology to be ready for introduction at the 14nm node, giving designers more freedom from the restrictive design rules that would be required for extensive double patterning. Gary Patton, vice president at IBM’s semiconductor R&D center in Fishkill, N.Y., said “power is the new issue” facing device makers.

For IBM’s 15/14 nm technology, where the pitch will be about 80 nm, IBM may be forced to rely on double patterning, a variable light source, source-mask optimization, and continued imposition of design restrictions. If EUV were to be available for critical layers, Patton said IBM and its Fishkill partners could “relax the constraints on the light source and the mask, and avoid the need for design restrictions and double patterning.”

“At the 11 nm generation, we will certainly need EUV,” Patton said.

Jim Clifford, senior vice president of manufacturing at Qualcomm, also said the industry needs EUV to continue scaling, which he defined as increasing the transistor density within a set power budget. Costs are the main worry. “Is it going to be cheaper to scale if lithography is half the wafer cost?”

During a June 30th briefing at the ASML facility in Wilton, Conn., ASML vice president Noreen Harned outlined ASML’s efforts to get EUV ready. At its Netherlands facility, ASML has “completely built” one NXE 3100 pilot production scanner, with “another one right on its heels.” While Harned did not disclose which companies will get the six NXE 3100 systems, she did say that ASML had more than six companies queuing up to order the 3100 scanners. ASML decided to limit the 3100 to six units so it could move on the NXE 3300B volume production scanner by the first half of 2012. (ASML’s Harned describes the NXE 3100 as a pilot production machine that can be used for early production.)

Boosting the source power to >350 Watts is key to acceptable EUV wafer-per-hour throughput. (Source: ASML)

With IMEC as the expected first customer, other likely recipients of 3100 systems include Hynix, Intel, Samsung, Toshiba, and TSMC. One 3100 customer is expected to reach wafer-out status by the end of this year.

ASML installed the Cymer laser-produced plasma (LPP) source on the first 3100, but ASML is working with other source vendors. At the Wilton briefing, Harned announced that in early July ASML achieved the “first light” milestone on the first NXE 3100 platform, with a thermally controlled housing and mechanical dynamic stability. However, she said the source currently provides only 30-40% of the power ASML needs to achieve the 60 wph throughput target for the 3100 systems. The source power can be upgraded in-place, through various techniques, she said.

While Cymer appears to have the inside track at ASML thus far, Harned said the ASML EUV NXE platform was designed to be source neutral. Japan’s Gigaphoton Corp. is working on an LPP source that Harned said is similar to Cymer’s but which takes “a different approach to debris mitigation.” Among the discharge-produced plasma (DPP) vendors, Ushio Corp., working at its Aachen, Germany site, has made the most public progress so far. “All three have plans in place to meet the scanner needs. They all have credible roadmaps to intersect with high-volume production,” she said.

Asked about cost-of-ownership considerations, Harned said the lifetime of the collector optics is one concern. Unless mitigation efforts are improved, debris from the interaction of the laser and the tin droplets can damage the mirrors used to collect the source power. “The collector optics are the most-expensive consumable. Our target is a year, but it must be six months at the start. Fortunately, the collector optics are refurbishable,” she said.

Responding to a presentation by TSMC’s Burn Lin at the IEDM last December, in which Lin claimed that power consumption of the EUV scanners would make them impractical, Harned said ASML’s data shows that an EUV scanner and source will consume about 3X the power consumption of a 193 immersion scanner. “Power consumption is dominated by the source, and to a lesser extent by the need for vacuum operation. But it is about 3X, and that is a real measurement,” she said.