To reassure its most important customers, ASML developed a membrane that protects EUV masks from microscopic debris.
There’s one misunderstanding that Dan Smith at ASML wants to clear up for once and for all: the pellicle, a protective membrane for the mask, isn’t necessary for EUV lithography. ‘Our machines are extremely clean and particles can’t move around easily in a vacuum. Our data reveal that the mask picks up most of the particles outside the machine,’ says the director of product management. ASML nonetheless began developing an EUV pellicle, because several customers asked for it. ‘Our customers see it as an insurance policy. If something goes wrong during production, the mask is protected.’
Most likely it’s the foundries who will sleep more soundly knowing there’s a pellicle available. Compared with memory manufacturers, they conduct far more and more varied production runs, which increases the chance of a mishap. Once a memory manufacturer has started up its process, it can essentially rest easy.
Studs
Though a pellicle is standard in DUV production, it wasn’t initially even considered for EUV. Every material absorbs EUV radiation, after all, and because EUV masks are reflective, the light has to pass through them twice. It’s hard enough to get sufficient light onto the wafer as it is.
But when in 2012 ASML achieved promising small-scale results using ultra-thin layers of silicon – which it had already explored as a spectral filter – customers encouraged the company in Veldhoven to keep going. As of 2016, two hundred employees are working on the pellicle, and its development has reached the industrialization phase. The first full-fledged product will let 88 per cent of the light through in each pass, at a maximum source power of 250 watts. ‘We’ve shown that the impossible is possible; now we’re going to make it an everyday reality,’ says Smith.
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The production version consists of a delicate polysilicon film some fifty nanometres thick that’s stretched on a frame. It measures eleven by fifteen centimetres and is coated on both sides with a layer just a few nanometres thick that protects it from thermal stress, EUV radiation effects and hydrogen present in the machine.
ASML is also working on the associated tooling. To inspect and clean a mask, the pellicle must be removable. So ASML and it suppliers have developed a machine to glue four T-shaped studs onto a mask, a machine that attaches the pellicle to the studs and takes it back off if required, and a machine that removes the studs.
Handling it all
The tooling draws upon ASML’s core competencies: it’s precision work to attach the pellicle to the mask. The mechanism has been designed so that no materials slide against one another, which could produce microscopic dust particles. There’s no contact until the very last moment, when the pellicle rests against the studs.
The pellicle itself, in contrast, is not the kind of technology that instantly brings ASML to mind. In fact, the company is notoriously hard to persuade to explore pathways outside its core business. But to make EUV a success, it’s making an exception for the pellicle.
That said, Smith notes, it isn’t easy to find a single company that can handle all the aspects of the pellicle. ‘On one hand you’re concerned with chemical, mechanical and thermal durability; on the other, there’s the optical performance: transmission, uniformity and defect density. All these things have to be balanced against one another.’
ASML would nevertheless heartily applaud others willing to enter the pellicle business. ‘We’ll gladly support them,’ says Smith.