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EUV Pellicles

EUV Pellicles
by Paul McLellan on 07-31-2014 at 8:01 am

Shakespeare reckoned that a man went through seven stages in his life.All the world’s a stage, And all the men and women merely players. They have their exits and their entrances, And one man in his time plays many parts, His acts being seven ages.

Well, an EUV mask seems to only go through three main stages:


  • the blank supplier manufactures and inspects the blank. This is a silicon-molybdenum multi-layer Bragg interference mirror
  • The mask shop patterns the mask, cleans and repairs and inspects it
  • The wafer fab stores it, puts it in the scanner, exposes wafers, and cleans it from time to time.

    I’ve written before about one of the other big problems for EUV beyond the light-source power issue, and that is the fact that currently EUV masks don’t have a pellicle. Just in case you have no idea what that means here is the quick explanation. In refractive masks (traditional and immersion lithography, the light passes through the mask) the mask has a thin transparent layer over the top of the mask known as a pelllicle. The stepper focuses its 193nm light so that the mask itself is the focal plane. If any contamination gets on to the mask it sits on top of the pellicle and is not in the focal plane, and unless it is gross contamination, it will not print. EUV masks are reflective (mirrors). There are lots of challenges to developing a pellicle system for EUV, largely being driven by ASML like all things EUV. One challenge is that almost everything absorbs EUV so you can’t just make a pellicle out of some random material that is transparent to normal wavelength light.

    How important is having a pellicle? ASML reported that users of the NXE3300 (ASML’s current EUV scanner) say:

    • Required for high-volume manufacturing (HVM)
    • Essential
    • Must for production
    • High level of interest

    At SEMICON there was a half-day meeting of the Industry EUV Pellicle Working Group. I didn’t attend it but I have been passed several of the presentations.

    The current state of the art is pSI pellicle with SiN caps on both sides in a frame, with 80% transmission. The table below has more details, accurate as of July 2014 for prototype pellicles.

    And this second table shows the status for HVM. Note that for some reason the targets and the status are in the other order from the prototype status table.

    Of course there is a lot more to bringing pellicles into HVM that requires a whole ecosystem, including inpection, cleaning, testing and so on. This table shows who the participants are in the various areas.

    At the end of the workshop, participants were asked when the predicted EUV insertion would take place. Most people said 2017. And on being asked what they thought the biggest problems were they picked light source, mask defectivity and resists in that order. So maybe everyone expects the pellicle issue to become a non-issue by then.

    More articles by Paul McLellan…

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