ASML and its partners had to overcome many challenges in
order to make EUV lithography a reality.
For instance, in a previous video I talked about the EUV light source and its double-shot
technique. But while challenging, that had not been considered one of EUV's dealbreaking issues.
A greater struggle was how to achieve a zero defect rate for the EUV photomask,
or reticle. I will use the two terms interchangeably here.
The EUV reticle contains the chip design. Any defects on the reticle larger than a certain size
will show up on the printed wafers themselves. So in order for it to work, it must be truly perfect.
In this video, we look at how ASML managed to overcome this
ultra-critical aspect of the technology.
## Meet the Reticle
The EUV reticle or mask is a mirror. On its surface is a
protective capping layer and a mirror layer - a multi-layer Bragg reflector
made up of 40-50 alternating layer pairs of molybdenum and silicon.
The mirror layer sits on top of a substrate of low thermal expansion glass. The essential point for
this substrate is that it must be extremely clean and stable over a wide range of temperatures.
The chip design data pattern is applied onto the mirror layer using 2-3 layers of
an absorber chemical - perhaps a metal compound of tantalum, titanium or nickel. So EUV light
reflects off the reticle like a pattern mirror and creates a cool reflection onto the wafer.
During use, the reticle is loaded into the EUV
machine and secured using an electrostatic clamp. The sensitivities are so high
that a particle several microns high can negatively affect the reticle's flatness.
As it turned out, the biggest technical challenge wasn't making a reticle mirror
that reflects enough EUV light. There are a number of other mirrors inside
the machine’s optical system and they have to be just as reflective.
But the reticle mirror needs to be more than just a regular EUV
mirror. It has to be a mirror beyond all EUV mirrors. Incredibly smooth and flat
so that it can properly reflect the data layered on its surface.
Defects print. Defects that print ruin wafers. No such defects can be allowed.
## Defect Types
There are four general types of mask defects - Particle,
Process, Absorber and Multilayer defects.
Particle and process defects are issues that can occur in the fab. As the name implies, these
occur either because of stray particles invading the mask or flaws occurring in the fab process.
Absorber defects are issues relating to the absorber layer on the reticle.
These are actually kind of interesting, but out of the scope of this video.
Fabs and technicians are familiar with these first three categories of errors.
Not to say that they aren't hard or present new issues, but people know how to deal with them.
But the fourth class of defect ... this is new.
## Multilayer Defects
Multilayer defects relate to the mirror layer and how it reflects EUV light. There are two
subcategories of multilayer defects - Amplitude Defects and Phase Defects.
Amplitude defects are particles or pits on or near the mirror layer's
surface. These can be created during any of the mask cleaning or polishing steps.
During the exposure step, amplitude defects absorb
EUV light, causing contrast issues between itself and the surrounding mirror areas.
Phase defects are more nefarious. These are bumps and defects on the surface of the mirror layer.
Many of these come from particles or pits on the substrate's surface
which then get layered over during the deposition steps.
Phase defects are arguably more sucky because they are very pronounced and as a result are very
printable. A phase defect as small as 1 nanometer in height or depth can print onto the wafer.
## Firing Blanks
There are two overarching processes involved in making an EUV mask.
First, you produce an EUV reticle "blank" that is free of information.
After that, you pattern it with the chip design - applying that
EUV absorber layer we mentioned earlier.
This second patterning step is done largely the same way as with traditional
193 nanometer photomasks - E-beams and all. I talked about this in a previous video.
While still challenging, most experts agree that the secret sauce is in making the blank.
Accordingly, you can break these processes down into about 30 named process steps. Applying the
absorber layer comes late in the sequence, right before Blank Protection and Packaging/Shipping.
The rest are related to making and processing the Blank or its substrate.
Cleaning steps alone make some 20% of the whole sequence.
The most ideal situation is that the EUV mask blank is made without any defects.
But while mask makers are getting better at making blanks,
there is a reason why zero-defect blanks are still referred to as "champions".
If you only accepted champions then your mask yield rate - the percentage of usable
mask blanks - would be abysmal and that would further raise the already-absurd cost
of EUV. For EUV to work, we need to be able to find and mitigate blank defects.
## Detect Detection
In the early days, we inspected for EUV mask defects using DUV light.
But the mirror layer is so reflective that such tools cannot detect defects
if they are located deeper than 2-13 layer pairs within the mirror layer.
So the only light capable of penetrating enough of those multilayers is the very EUV
light used by the system itself. This is called "actinic" light - the system's active wavelength.
Using actinic light means having an actual EUV light source available to inspect a blank or mask.
There are many issues associated with this method,
but actinic blank inspection is logically the most credible way to inspect the mask blanks.
It took many years to develop these inspection tools. The most notable of which was developed by
an alliance of two Japanese companies - Lasertec and Eidec - based on a concept advanced by Japan's
National Institute of Advanced Industrial Science and Technology.
This is a large tool over 4 meters wide and 5 meters long.
It is like a little EUV lithography machine all by itself.
There is an EUV light source that reflects off a set of EUV mirrors
and illuminates onto a small portion of the blank - about a square micrometer large.
The blank naturally reflects the light back. If there is no defect,
then all of the reflected light stays in what is called the "bright field".
But if there is a defect,
then that defect scatters the light - causing it to enter what is called the "dark field".
If your dark field scanners pick up any EUV photons, then you know your blank has a defect.
Lasertec's pioneering actinic blank inspection
tool - the ABICS E120 - caps 20+ years of research and development. The tool
received various awards throughout 2017 and 2018 for its breakthrough nature.
They aren't perfect. Notably, they can't inspect the substrate surface
where many phase defects originate. Nevertheless, these tools are a critical reason why the industry
has been able to drive down the number of mask blank defects over the years.
## Defect Repair and Mitigation
Once you find a small defect, the ideal outcome would be to fix it.
But how do you do that? Unfortunately, there remains a lot to do here in this field. Different
defects require different repair methods. Let's go back to our original definitions.
Amplitude defects are bumps on the surface due
to particles embedded near the mirror layer's surface.
So repairing the defect means taking out the particle,
thereby restoring the mirror layer's reflectivity. Studies have found that you
can take out up to 20 mirror pairs and only lose an acceptable amount of reflectivity.
This can be done with either a Focused Ion Beam - which is
a real thing and not something out of Star Wars - or e-beams.
Phase defects start on the substrate, at the bottom of the mirror layer stack. These are
far more challenging to fix, and they haven't come across a good method yet.
I read a study published in 2002 where they tried banging a phase defect's bump back down.
It sounds like something out of a Saturday morning cartoon, and as expected it did not really work.
With EUV, you have a great deal of smaller defects.
So finding a tool that can fix even the smaller ones is essential.
Probably the current industry standard tool for doing this is Carl Zeiss's
photomask repair tool MeRiT - which uses a low voltage electron beam to fix lines.
And finally, one last thing you can try is mitigation. Defects on the blank interfere
with reflectivity. So why not make it so that reflection does not matter?
Reposition the chip design in such a way that you put the absorber layer right on top of the defect!
This is called pattern shift and it works. But special EDA software is needed to pull it off
as well as knowledge of the defect's exact shape and nature.
But again that is possible thanks to Actinic Blank Inspection.
## The Pellicle
Older lithography employs the use of something called a pellicle.
It is a thin organic film that physically stretches over a metal frame about 2.5
millimeters on top of the patterned mask to protect it from stray particles.
IBM invented the concept back in 1977 and the industry quickly adopted it a few years later.
Traditional DUV pellicles were made of thin polymers 99% transparent to radiation.
But for EUV pellicles, engineers had to reckon not only with the fact that most materials
strongly absorb EUV light, but also that the light has to go through the pellicle twice.
Once before it hits the mirror, and a second after it reflects off it.
This doubly reduces the EUV light intensity by 81% or 90% squared.
The weaker the EUV light, the less throughput the EUV machine will have. That in turn raises the
amount of capital expenditure depreciation per wafer - making them more expensive.
Starting in 2006, ASML led a widespread search - kind of like a Britain's Got Talent - for a
suitable EUV pellicle. Finding that pellicle turned out to be one of the more difficult,
technically challenging aspects of achieving EUV.
## The Search
The first serious candidate for the task was again silicon. Some silicon compounds' abilities
to absorb EUV light - the fancy term to use is "extinction coefficient" - is relatively low.
Scientists were able to fabricate silicon-based honeycomb mesh membranes
about 10 nanometers thick - a remarkable achievement by itself.
At those thicknesses, it would be theoretically possible to reach the 90%
EUV light transmission threshold.
However, researchers discovered that silicon meshes become fragile
after they get exposed to high powered amounts of EUV light
and heat. When that happens, they start to wrinkle. Research was eventually halted.
People then looked at graphene, a carbon-based nanomaterial that can do everything except
leave the laboratory. 100 times stronger than steel, it has high thermal tolerances and is
very transparent to EUV light. But ASML could not grow enough of it without serious defects.
## Two Paths and a Winner
When ASML first demoed its Alpha Demo Tool in 2008,
no suitable pellicle had yet been discovered that met all the requirements.
So ASML decided to take a dual approach with their next few scanners - a more
traditional system with a pellicle and a "clean system" without one.
The latter required extensive work tracing through the entire supply
chain to make sure no particles were added during assembly and shipment.
Additional engineering work would be needed to prevent
EUV plasma particles from reaching the mask.
And we need to do a great deal more inspections, which means less time for making wafers. Super 麻煩.
The best pellicle at the time was from the aforementioned silicon approach - a thin film
made up of poly silicon and silicon nitride that ranges from 25 to 55 nanometers thick.
It kinda met ASML's needs, but in a 2016 paper
ASML was still encouraging industry partners to find a better solution than what they had.
Luckily however, imec and others eventually hit upon a suitable solution:
Carbon nanotubes. First investigated in 2015, these are carbon atoms rolled into a tube. They
are 96.5% transmissible to EUV light and are 10 times stronger than traditional fibers.
The pellicle is made up of several nanotube layers.
You mix them together into an ink and spray them onto a surface.
Because of this, they can be used to tune the properties of other materials.
For instance, adding carbon nanotubes to the aforementioned
silicon nitride pellicle gives it increased strength and thermal conductivity.
Or just use a free-standing solution made up purely of carbon nanotubes.
They are extremely customizable which is exactly what ASML was looking for.
## Conclusion
EUV stretched the very limits of what many thought was possible.
What ASML and the rest of the global semiconductor industry managed to achieve here,
I am reminded of a Elon Musk quote during an Everyday Astronaut video:
"[At SpaceX] we specialize at converting things from impossible to late".
EUV was late. Up to 15-20 years late. But the amazing thing to me
was just how much transparency they gave throughout the whole process.
During their presentations, ASML explains their technical challenge, their final goals,
and presents their expected milestones towards it.
In the lithography and semiconductor world today, there is no A-HA! moment.
Instead, it is a long, slow trudge. But that is what makes the final achievement so special.