This year the Advanced Lithography Conference felt very different to me than the last couple of years. I think it was Chris Mack who proclaimed it the year of Stochastics. EUV has dominated the conference for the last several years but in the past the conversation has been mostly centered on the systems, system power and uptime.

I will be writing up more detailed blogs from interviews with ASML and imec and their presentations, but I wanted to first present some overall impressions.

ASML is now delivering systems with acceptable power (at least for initial use) and uptime is improving, although not there yet.

The conversation about EUV has now shifted into the practical details of the process parameters and there are lots of details to work through!

Perhaps the key issues remaining with EUV relates to dose and line edge roughness (LER). Dose is the number of photons delivered multiplied by the photon energy. because EUV photons are so much more energetic than deep UV (DUV) photons there are 18 times less photons in EUV for the same dose. The small number of photons leads to shot noise at low doses, but this is just the tip of the iceberg.

The random defects from EUV exposure can lead to micro bridges and open lines for line/space pairs and missing or bridged contacts in dense contact arrays. In his Keynote address Yan Borodovsky noted that complex designs can have up to one billion vias and a single bad via can kill the circuit. Under this condition even a five-sigma process will result in zero yield!

At Litho Vision held the Sunday before SPIE, John Biafore of KLA Tencor noted that current EUV photoresists only absorb about 20% of the incident EUV photons. Ideally a photoresist should absorb around 45% of the photons to maximize sensitivity while insuring even exposure down through the depth of the photoresist. This presents an opportunity to double the sensitivity of the photoresist doubling the effective dose for a given delivered dose. Of course, if this was easy it would already be done. The photoresist needs to absorb EUV photons and generate secondary electrons with the correct characteristics that trigger photo events.

In Patrick Naulleau’s talk “The implications of shot noise on EUV patterning” he made several interesting observations:

LER goes up exponentially at lower does and even at high dose it is never zero with around 1.5nm as a minimum.

For chemically amplified photoresists there are five reaction variables that determine LER:

[LIST=1]

Photon shot noise (RV1)

Photons generate electrons that generate photo acids (RV2)

Acids are only generated where you have PAGs (RV3)

Reaction diffusion (RV4)