Should lifetime of EUV optics be a concern?

It’s well known that EUV adoption is running later than hoped, mostly due to inadequate source power (although ASML and Cymer say they are on track to provide workable solutions and imec says it’s on track for the 10nm node). After that, the main challenge could be those associated with EUV mask blanks, which are essentially sophisticated mirrors. The dual challenge there is that they are not only difficult to produce without defects, but they are difficult to inspect. Presently, the only way to really test them is to fabricate them and see what kind of pattern results after they’re used.

But another challenge recently came to my attention: the optics in the EUV system, which are also sophisticated mirrors made of multi-layer structure, get contaminated during operation. This degrades their quality over time, and eventually the system must be disassembled and the optics recoated or replaced.

I recently talked to Dr. Harro Hagedorn, head of R&D at Leybold Optics in Alzenau, Germany. Located about 20 minutes outside of Frankfurt, the company supplies evaporation and magnetron sputtering systems used to fabricate the multi-layer coatings used for EUV collector optics and or many other applications such as synchrotron labs and X-ray devices. And they work with ASML and Zeiss, and the Fraunhofer Institute.

Speaking on EUV, Dr. Hagedron, said the output of the light source is still not high enough, and also the lifetime of the optics was a concern. “This light source is normally an awful thing for the optics because you have a metal droplet that is heated up by a laser and then it creates a plasma. These metal droplets are also contaminating the optics,” he said. To correct, this “they have to disassemble the system and recoat these optics. They are very expensive. Also, the life throughput that comes from this light source and through the optics goes down. They have to also manage this,” he said.

Part of the complexity and expense of the optics is that they rely on interference coatings that require stacks of layers. “The challenge is that these layer stacks are incredibly thin, 3-4nm, with coating uniformities in the range of 0.1%,” Hagedron said. “It’s not any more than a diameter of an atom.” The goal for these optics is a reflectivity of nearly 70%.

Investigating a bit further, I found that there has been a significant amount of research into the lifetime of EUV optics. In fact, earlier this year in April, a session at SPIE was dedicated to damage to VUV, EUV, and X-ray Optics. One of the papers by Laser-Lab in Germany, KLA-Tencor and Fraunhofer, described work that characterized EUV damage thresholds and imaging performance of Mo/Si multilayer mirrors. Here’s a summary:

Currently, more and more powerful EUV sources for next generation semiconductor microlithography are being developed, for which novel optical elements like multilayer or grazing-incidence mirrors are required. Consisting of very thin alternating layers, especially molybdenum and silicon for the wavelength of 13.5 nm, multilayer mirrors are typically employed for near-normal reflection angles. These mirrors are presently being optimized with respect to thermal resistivity and reflectivity. However, only very few ablation and damage threshold studies at a wavelength of 13.5 nm are available up to now for these optical elements.

We studied 1-on-1 and 10-on-1 damage thresholds of Mo/Si multilayers with EUV radiation of 13.5 nm wavelength, using a table-top laser produced plasma source based on solid gold as target material. The experiments were performed on different types of Mo/Si mirror, showing no significant difference in single pulse damage thresholds. However, the damage threshold for ten pulses is ≈60 % lower than the single pulse threshold, implying a defect dominated damage process.

Using Nomarski (DIC) and atomic force microscopy (AFM) we analysed the damage morphologies, indicating a primarily thermally induced damage mechanism for higher fluences. Additionally, we characterised transmission and reflection properties of novel Mo/Si multilayer beam splitters performing wavefront measurements with a Hartmann sensor at 13.5 nm wavelength. Such wavefront measurements allow also actinic investigations of thermal lens effects on EUV optics.

My main takeaway from all of this is that even if the technical challenges of EUV make it ready for production for the 10nm device generation, there still must be lots of questions about the ultimate cost of ownership and how that will compare to double and triple patterning approaches with 193nm immersion.