The devilish details of EUV lithography

By Ed Korcynzski

Industry R&D consortium imec runs a series of technology forums around the world, starting in June in Antwerp, Belgium, and including a stop in July in San Francisco in coordination with SEMICON West. Greg McIntyre, imec Director of Advanced Patterning, discussed the state-of-the-art in Extreme Ultra-Violet (EUV) lithography technology with Solid State Technology during the Antwerp event. While still focusing on “path-finding” R&D for industry, the recent technology challenges associated with commercializing EUV lithography has pulled imec into work on patterning ecosystem materials such as resists and pellicles.

With each NXE:3400B model EUV stepper from ASML valued at US$125 million it costs $1 billion to invest in a set of 8 tools to begin high-volume manufacturing, and the entire lithography materials supply-chain is engaged in improving availability and throughput of this expensive tool-set. For high performance logic ICs we need EUV to reach the smallest and most powerful FETs possible, so EUV is in pilot production for logic chips at Samsung and TSMC this year, and will likely begin pilot ramps at Intel and GlobalFoundries next year.

The first use of EUV in IC HVM will be as “cut-masks” for use in self-aligned multi-patterning (SAMP) process flows that start with argon-fluoride-immersion (ArFi) deep ultra-violet (DUV) steppers. Such a first use allows for substitution of three ArFi “multi-color” cut-masks in place of the one EUV mask, in case there are unanticipated issues with the new EUV steppers. Second use in HVM will then happen using a single-exposure of EUV to pattern metal layers, but with no ability to use multiple ArFi exposure as a back-up.

“We will not put EUV in our critical path,” commented Dr. Gary Patton, GlobalFoundries’ CTO and SVP of Worldwide R&D, during a presentation in Antwerp, “But it’s clear that it’s coming and it will offer compelling advantages.” Patton said the company is experimenting with two of ASML’s EUV steppers in a New York fab, and will launch the company’s “7-nm-node” finFET production first with ArFi and then move to EUV when the throughput and uptime of the process make it affordable in their cost models.

Figure 1 shows the extremely small patterning process window around 18nm half-pitch line arrays (P36) using EUV lithography with Dipole source-mask optimization (SMO):  micro-bridging between lines starts below 15.5nm, while breaks within lines start above 18nm. These stochastic failures (Ref:  “Waddle-room for Black Swans:  EUV Stochastics”, are caused by variations in the photons absorbed by the resist (a.k.a. “shot noise”), the quantum efficiency of photo-acid generation (PAG) and diffusion, thequencher distribution,and optical and chemical interactions with under-layers for adhesion, anti-reflective coatings, and hardmasks.

Figure 1. Stochastic failures due to atomic-scale variability are shown in top-down CD-SEM images taken from 36-nm Pitch (P36) line/space arrays of post-etched photoresist that had been patterned using EUV lithography, which define the limits of the patterning process window when plotted as Percent Not-OK (%NOK) within an inspected area. (Source: imec)

Every nanometer of resolution is difficult to achieve when patterning below 20nm half-pitch, with many parameters contributing noise to the signal. For EUV lithography using reflective optics, the mask surface causes undesired “flare” reflections from the un-patterned area, such that bright-field masks inherently distort images more than dark-field masks. Since cuts typically only expose <20% of the field, these masks will be much less noisy as dark-fields.

Given the need for dark-field cut-masks, the ideal photoresist will be positive-tone (PT) which means that reformulations of Chemically-Amplified Resists (CAR) based on organic molecules can be used. Standard organic CAR tuned for ArFi lithography provides some sensitivity to EUV, and blends of standard CAR molecules can be tuned to improve trade-offs within the inherent Resolution, Line-Edge-Roughness (LER), and Sensitivity trade-off triangle. Consequently, all of the suppliers of ArFi CAR are capable of supplying some EUV CAR. Since stochastic effects are interdependent, resist vendors have to explore integration options within the entire stack of patterning materials.

JSR co-founded with imec the EUV Resist Manufacturing & Qualification Center NV (EUV RMQC) in Leuven, Belgium, where an EUV stepper at imec is available for experiments. “RMQC is running at full speed, and shipping out production lots,” said McIntyre. “Intel’s Britt Turkot mentioned at SPIE this year that the resist qualification work being done at IMEC has been very beneficial.”

ASML now owns the critical-dimension scanning-electron microscopy (CD-SEM) technology of Hermes Microvision Inc (HMI), and Neal Callan, ASML’s Vice President of Pattern Fidelity Metrology, spoke with Solid State Technologyabout controlling EUV patterning. Electron-beams cause shrinkage in organic films like CAR, and that shrinkage results in a CD bias that can be more than one nanometer. Different CAR formulations from different vendors shrink at different rates, and the effect is more difficult to model in 2D structures. ”We’re being pushed for accurate metrology in terms that can be quantified,” explained Callan. “The biggest issues are in terms of CD-bias with 2D features. We need to build more accurate models to create better data for OPC and for computational lithography, and also for our etch modeling peers.”

“Design rules for EUV need to be stochastically aware,“ confided McIntyre. “Designers need to know how much can be sacrificed in a design rule such as tip-to-tip spacing depending on the pattern pitch. There are different ways that we can think about minimizing stochastic effects.”

While stochastics and systematic yield losses increase in relative importance with decreasing device dimensions, losses due to random defects are also more difficult to control. Figure 2 shows second-generation EUV pellicles made from carbon nano-tubes (CNT) by imec to protect EUV masks from random particles while transmitting ~95%. First-generation pellicles reportedly transmit <90%.

Figure 2. Second generation EUV pellicles based on carbon nano-tubes (CNT) demonstrate increased transmission of ~95% while maintaining sufficient mechanical stability to protect reticles. (Source: imec)

“Today, new purity challenges are not only faced by the fab but also by their materials suppliers driving sharp increases in the use of filtration and purification systems to prevent wafer defects and process excursions,” explained Clint Harris, Senior Vice President and General Manager, Microcontamination Control Division, Entegris, to Solid State Technology.“The transition from 45nm- to 10nm-node has resulted in a 2.5x increase in the changeout frequency of filters as well as a 4x reduction of maximum allowable contaminant size. This trend is expected to continue as device parametric performance becomes more sensitive to particles, gels, metals, mobile ions, and other organic contaminants.

[As a TECHCET Analyst, Ed Korczynski writes the TECHCET Critical Materials Report (CMR) on Photoresists & Ancillaries.]


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2 thoughts on “The devilish details of EUV lithography

  1. PSM

    K1=0.44 darkfield requires attenuated phase shift mask, which has not been developed for EUV yet.

  2. Frederick Chen

    The Line breaking events in Figure 1 are probably caused by secondary electrons from adjacent exposed areas, which increase with wider CD (shorter distance for secondary electrons).

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