As feature sizes shrink and device densities increase, ensuring that masks remain defect-free becomes critical.
In the autumn, I had the opportunity to attend the 2024 SPIE Photomask Technology and EUV Lithography conferences, collectively referred to as PUV or sometimes BACUS, the latter a reference to the event’s early association with the BACUS organization. This is a key annual event that brings together experts and professionals in photomask technology and EUV lithography. This year’s conference was particularly exciting, with a palpable sense of optimism and enthusiasm about the future of the semiconductor industry, which is performing well overall.
The positive mood was driven by strong growth in the semiconductor market, particularly the photomask sector, as well as good progress in the newest leading-edge lithographic technology, high-NA EUV lithography. During the annual eBeam Initiative reception, Aki Fujimura shared a graph showing photomask revenues over time, which have been strong since 2017. This positive business environment and technical progress (discussed further below) set the tone for the week.
I think that there had previously been some apprehension regarding high-NA EUV lithography, given prior difficulties in getting EUV lithography into high-volume manufacturing in the first place. However, one of the plenary speakers, Christophe Fouquest (CEO of ASML), put this nicely in perspective. There were certainly some fundamental and very difficult problems with EUV lithography that needed to be solved, but once solutions to these problems were found, new lithographic technologies could now advance on a more even cadence.
As always, one of the best aspects of attending in person was the chance to connect with colleagues and have in-depth discussions on the latest developments in lithography and photomask technologies. It’s always valuable to exchange ideas and share insights, especially as we push the boundaries of what’s possible in semiconductor manufacturing.
Nowadays, leading-edge lithography means EUV lithography, and there was a significant focus on EUV mask-related issues, such as the ongoing challenge of mask defects. As the industry continues to shrink feature sizes and increase device densities, ensuring that masks remain defect-free becomes critical. One major area of progress has been pellicles for EUV lithography. Pellicles, long used in optical lithography, are protective membranes that shield masks from contaminants and defects during the lithographic process, and while they’ve been under development for EUV for years, we’re now seeing a broader adoption of this technology as it matures. The expansion of pellicle use is helpful as we continue pushing for smaller features and denser devices, and it’s clear that this technology is poised for broader implementation in the near future.
High numerical aperture (NA) EUV lithography is another key area of focus for the industry. The move to high-NA systems promises to enable even smaller features in semiconductor manufacturing, but it also brings with it new challenges. For one, high-NA systems have an exposure field that’s only half the size of traditional EUV and optical exposure tools. This means that for large chips, you need to use two separate masks to expose the entire chip, which requires precise stitching at the boundaries between the two exposures. While this presents some significant challenges, it was encouraging to see the progress being made in overcoming these hurdles. At the conference, several presentations highlighted promising solutions for stitching.
Another solution for patterning large chips is to use larger masks. In conjunction with the conference, there was a workshop on large format masks, with over 150 attendees this year, up from around 70 last year. This shows the growing interest in large format masks, yet it’s fair to say the industry has not yet made a commitment.
There were several presentations on multi-beam mask writers, which are enabling the fabrication of masks with very small features, good linewidth control, and low line-edge roughness. All of these are needed for future nodes, which will involve masks with sub-resolution assist features that must be very small (in order not to print when using high-resolution, high-NA EUV exposure tools). Both suppliers of multi-beam mask writers gave presentations on their newest models that have enhanced capabilities compared to prior generations of mask writers. Users of multi-beam mask writers also gave presentations on the use of these tools. One interesting strategy shown for writing masks involved applying high exposure doses selectively to the edges of features, which improves edge acuity. In one embodiment of this approach, pixel-level dose correction, the doses can be adjusted in real time as masks are written, obviating additional cycle time for data preparation.
One of the ongoing challenges in advanced lithography is the need for curvy features on photomasks. Techniques like source-mask optimization and inverse lithography are increasingly being used to optimize process windows and enable packing at higher densities. It has long been known that optimal solutions produced by these techniques involve curved, rather than straight, mask features. In the past, patterning curvy features on masks using variable-shaped beam (VSB) mask writers involved impractical write times. Multi-beam mask writers solved this problem, but much additional infrastructure that is needed to support such features is still developing. There were many papers presented at the conference that described the progress being made in this area. It’s clear that the necessary infrastructure is maturing, and I expect that the use of curvy features will become more widespread in the near future.
As always, EUV resist technology was a key topic of discussion at the conference. The chemistry of EUV resists differs from that of optical lithography in that the resist radiation chemistry is not driven directly from the absorption of EUV photons but from the resulting photoelectrons and secondary electrons. Understanding how these secondary electrons move through the resist is crucial for improving the performance of EUV resists, particularly as we push for smaller nodes and better resolution. It was good to see papers showing the results of recent research in this area.
There was also a presentation on vertically tailored resists, a new approach for EUV resists in which the resist molecules stand upright on the substrate. This structure potentially allows for better control over the resist’s properties, such as reduced line-edge roughness at lower exposure doses and better selectivity in the etching process. This technology is still in development, but it holds great promise for the future of EUV lithography.
Another notable aspect of the conference was the increased attention on sustainability in lithography. The semiconductor industry is facing challenges related to environmental concerns, particularly in the area of resist materials. It was encouraging to see one company presenting a PFAS-free electron beam resist that demonstrated promising performance while addressing some of the sustainability concerns associated with traditional resists.
In conclusion, the 2024 PUV conference highlighted the significant progress being made in EUV lithography and photomask technology. From the increasing use of pellicles to advancements in multi-beam mask writing and high-NA EUV lithography, it’s clear that the industry is tackling the challenges of future nodes and more complex devices head-on. Moreover, this is being accomplished while considering sustainability issues. It was a great conference, and I’m looking forward to seeing continuing progress in the coming years.
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