For a long time, the semiconductor industry's primary economic engine was Moore's Law. An
internal benchmark of doubling the number of devices on an integrated chip every 18 months.
Broadly speaking, three engines drove these advances. Semiconductor design,
increasing wafer sizes, and lastly, lithography.
Improvements in optical lithography techniques have been the true driving force
behind producing faster and faster chips. But coming up to the new millennium,
it became clear to everyone that the lithographic train of progress was braking to a slow halt.
Was there enough left in the tank for one last ride?
In this video, we look back at what would have been the last
optical lithography technology. And why the industry failed to sign on.
## The Fall of Lithography
In the two decades since 1980, semiconductor lithography tools had the benefit of using
wavelengths smaller than or at least equal to the minimum feature size of their process nodes.
For instance, in 1980 the industry used g-line mercury arc lamps. These had a 435-nanometer
wavelength. The leading process node at the time was the 1.5 micrometer process,
corresponding to a 1,500 nanometer half pitch.
1,500 nanometers versus 435 nanometers.
In that situation it's like drawing thick lines with a sharp pencil, so much easier.
Then in the second half of the 1980s, the industry moved to the I-line at 365 nanometers.
Again, not a problem since the leading edge process node at the time
was 800 nanometers. But the pace of Moore's Law was starting to catch up on the industry.
In the late 1990s, the industry switched from using lamps to excimer lasers.
In 1999 - the year of the Dreamcast - the industry started using 248 nanometer KrF
lasers for their leading edge process node at the time:
180 nanometers. 193 nanometer ArF lithography was scheduled to come online two years later in 2001.
But by now Moore's Law was flying like the Millennium Falcon.
In just 13 years from 1987 to 1999,
the industry's leading edge process node went from 800 nanometers to 180 nanometers.
From the early 1980s, the minimum feature size on an integrated chip
had basically shrank about 100 times.
Then in 1999, it became the case that the leading edge process node had a minimum
feature size smaller than the most advanced lithographic wavelength. And from then on,
it became much harder to push the limit on semiconductor manufacturing.
## Next Generation
At around then, the semiconductor industry was putting the finishing
touches on 193 nanometer ArF laser technology - scheduled for a full launch at around 2001.
But with how fast Moore's Law was compounding now, analysts predicted that the 193 nanometer lasers
would be unsuitable for the 100 to 70 nanometer process nodes.
Those nodes needed to come out around 2003, less than 2 years after the lasers’ rollout.
But that's alright because help was on the way ... right?
By then, the industry had agreed that Extreme Ultraviolet Lithography or EUV
was their long-term future. The foundries wanted EUV equipment in their cleanrooms
for the 100 nanometer process node - just in time to replace those 193 nanometer lasers.
Unfortunately, it became clear that EUV was not ready. It was a completely new
technology that used mirrors instead of lens. The EUV light source was woefully underpowered,
the mirrors weren't flat enough, and there were serious defect issues.
Analysts predicted that EUV wouldn’t be ready until at least 2010. In actuality
it was far longer than that. So what can foundries use between 193 nanometers and EUV?
That is how the idea of 157 nanometers came about.
## Introducing 157
157 was meant to be a bridge from 193 to EUV - especially during
the 100 to 70 nanometer nodes which EUV should have been available for.
The difference between 193 and 157 nanometers was just 36 nanometers, but this 20% reduction
represents a bigger scaling down than from 436 to 365.
Furthermore, 157 nanometer lithography would use a fluorine excimer laser,
and thus was presumed to be similar to the 248 and 193 nanometer lithography processes.
Presumably, this would smooth out the transition since relatively little would change.
Considering the daunting challenges up ahead in future nodes,
some advocated for rushing this technology into cleanrooms to help keep Moore's Law alive. 157
equipment would hit cleanrooms in 2003 - just in time for the 100 nanometer process node.
But such transitions are never so easy to pull off.
## Optical Lithography
I want to pause here to briefly discuss with you optical lithography technology - the
magical light train that took us so far over a span of 30 years.
First the “lithography” part and then the “optical”.
Lithography is a process involving three system
parts - the Exposure Tool, the Mask, and the Resist.
First, you coat a wafer in a photosensitive chemical called a resist or photoresist.
Then you take a master image printed onto what is called a mask
and send high-powered light through that mask onto that wafer.
The system that does this is called the exposure tool - and inside that
would be your excimer laser, crazy multi-lens optical system and so on.
The resist reacts to the light so that the chip pattern gets printed onto the wafer.
After that, the pattern can then be used as a stencil for further etching processes.
That's lithography in a nutshell.
So why am I calling it "optical" lithography and does it matter? Optical lithography is different
from its next generation counterparts. It has 3 differentiating components.
First, it uses photons.
Second, its masks are transmissive photomasks. Meaning that they are transparent.
Third, they employ projection optics to shrink an image.
The pattern on the mask is often four to five times larger than what you want to put
onto the chip. This is a helpful benefit that makes the masks easier to create.
## Developing 157: Laser
Development began in 1997 with a micro-stepper prototype at the MIT Lincoln Laboratory.
It was far smaller than a commercial system, but it was capable of patterning
80 nanometer-wide lines, more than sufficient for the 100 nanometer node.
As I mentioned, a lithography machine's critical elements are the exposure tool,
mask, and the resist. All three would need extensive re-engineering for 157.
Let us start with the exposure tool, which contains the laser and the optics system.
Because ordinary air easily absorbs 157 nanometer light, the entire light pathway has to be purged
of air and replaced with inert nitrogen. This added substantial cost to the whole system.
For the laser, industry analysts early on worried whether the 157.63 nanometer
fluorine gas excimer laser could output enough power as its 248 and 193 generation peers.
Surprisingly though, laser suppliers quickly caught up
and started producing lasers with 10 and even 20 watts of power.
They were ahead of expectations, so this was essentially considered on track.
## The Optics System
But there was one serious problem with the laser. The width of its lines were too wide. Usually,
this is not a major problem. You can correct it at the optics system level with different
glass materials and leveraging how they disperse light - just like a pair of glasses.
Unfortunately, 157 nanometer light is so easily absorbed
that the optics systems designers - again, our friends at Carl Zeiss - only
had one option for the lens material in the whole system: Crystalline calcium fluoride.
Specialty glass companies like Corning and Schott started working on producing enough
of this crystal at extremely high purities - and the struggle was real. Furthermore,
they have to grow it in such a way to minimize what is called stress-induced birefringence.
Birefringence is a fancy word that describes how light splits into two
when it enters a crystal. This would scatter 157 nanometer light and reduce its contrast,
resulting in image aberrations. Image aberrations are not acceptable in semiconductor manufacturing.
When the crystal is deformed due to weaknesses in production,
unwanted birefringence occurs. The glass companies overcame this
but in 2001 - a few years into test production - they realized that the crystal inherently produced
some amount of birefringence no matter how unstressed and chilled out it was.
This intrinsic birefringence as it was called was measured at just
11.2 nanometers for each centimeter of crystal but nevertheless was a serious issue.
Zeiss and ASML engineers played with the idea of introducing a second lens material into
the optics system. Barium fluoride crystals - which occurs in nature as the amusingly named
frankdicksonite - have a similar intrinsic birefringence, but in the opposite direction.
However, the supply chain was not prepared to ramp up on this new material - they
were having enough headaches with calcium fluoride alone - so the idea was dropped.
Eventually, the engineers discovered that the amount of intrinsic birefringence
depended on the direction at which the light enters the crystal. So
they created crystal orientation pairs that canceled out each other's light distortions.
For instance, one crystal lens positioned in a specific orientation - which ASML labeled as
"111" - needs to be paired with two "100" orientation crystal lens. The
second brother of those two "100" lens has to be tilted 45 degrees in relation to the first.
This pairing requirement had knock-on effects throughout the rest of the system.
For instance in the optical lens coatings and
notably in the machine's embedded software - which needed rewrites.
But this crazy intrinsic birefringence compensation scheme
worked. Zeiss himself would be proud.
## The Mask
You might be wondering why the lithography industry chose this
specific wavelength - 157 nanometers. The number is very significant.
193 nanometers might have been the last optical lithography technology put into mass production.
But 157 is the last feasible optical lithography technology
period, and that is because of the photomask.
The mask is the semi-transparent template that holds the chip design.
For decades, the industry made their masks out of fused silica - pure silicon dioxide glass.
The industry preferred fused silica for this because it
handles big temperature changes very well.
If the temperature moves by 1 degree Celsius,
the glass's structure will only shift by about 0.00005%.
So if the chip pattern sitting on this fused silica plate is made up of lines
about 50 millimeters wide (remember the mask's features will shrink so
it doesn't have to be as small as what's on the chip), then 1 degree Celsius of
variation will cause it to distort by only about 25 nanometers. Acceptable.
157 nanometers is the last wavelength for which fused silica remains transparent. And as it
turns out, even the normal version absorbs 157 nanometer light. ASML and Corning had to develop
what it called a "dehydrated fused silica" that allowed just 85% of the UV light to pass through.
Using other materials to make the mask - like the crystalline calcium fluoride used
for the optical lens - was not viable because those materials would greatly warp and distort
during the wafer exposure stage. So the dehydrated fused silica
invention was a critical breakthrough allowing 157 masks to move forward.
By 2002, ASML was still figuring out how to produce a suitable pellicle - which
is kind of like a protective sheath - to protect the mask from contaminants,
but the component looked on track for mass production.
## Resist
If you recall, the resist is meant to help hold the chip pattern onto the wafer
by absorbing UV light from the mask. The biggest challenges here were chemical ones.
Conventional resists used for 248 and 193 were basically modified hydrocarbons.
But hydrocarbons at their conventional thicknesses absorbed too much 157 nanometer light.
So you either made the resist layer thinner - about 60
or 90 nanometers - which had accuracy consequences, or find something else.
Some of these newer materials are very exotic and were synthesized from known carcinogens. But more
importantly, it seemed unlikely that the industry would be able to formulate and commercialize
a proper resist alternative in time for the 90/100 nanometer process node generation.
They would just have to make the resists thinner. And that was where progress was when 157 ended.
## Stall and End
The teams developing 157 nanometer lithography managed to overcome many of these daunting
technical challenges. In the end however, they failed a more fundamental challenge.
In the early 2000s, 193 nanometer immersion or 193i
technology rapidly emerged as a legitimate next-generation candidate to succeed 193.
Immersion lithography involves sending UV rays through an immersion fluid
in order to improve the machine's printing resolution.
The industry suddenly found itself trying to support three viable next-generation
lithography technologies: 193i, 157, and EUV. There were too many options.
SEMATECH - North America's semiconductor industry collective - called in the leading representatives
for each of the three technologies to defend their approach in a Shark Tank-style bakeoff.
157 emerged as the weakest link. It was conservative,
but not as conservative as 193i - which did not require as much new technology.
And crucially a commercially suitable 157 immersion fluid had not yet been found,
while for 193i you can use simple ultrapure water.
And while 157 nanometer technology was coming along,
it was not coming along fast enough to roll out by the 100 nanometer process node. Crucially,
yields on acceptable amounts of crystal calcium fluoride for the lens were still in the 1% range.
And finally, there was the cost factor. Lithography machines represent over half
of the total capital costs of a fab. 157 nanometer machines were estimated to cost
twice as much as their 193 nanometer counterparts, a really hard pill to swallow.
In May 2003, Intel - one of the technology's
original champions - announced that they were dropping 157 out of their roadmap.
They cited technical factors for their decision, not economic ones.
The industry lurched on a little bit longer after that massive blow. They still held the 4th
annual International Symposium on 157 nanometer lithography in Yokohama that August, for instance.
And ASML shipped their prototype MicraScan
VII to their test customer and frequent collaborator IMEC in September 2003.
But within a year of Intel's big announcement,
the other semiconductor manufacturers also halted their development on 157 technologies.
## Conclusion
Suppliers of lithography tools, optical materials, photoresists, anti-reflection coatings, pellicles,
masks, mask blanks, metrology tools, and inspection tools.
Those guys had collectively invested hundreds of millions of dollars in developing 157 nanometer
lithography technologies. Now flushed down the toilet. The industry pivoted to 193i in the
short term and EUV in the long term. ASML released its first immersion Lithography TWINSCAN in 2006.
Optical lithography gave the semiconductor industry its golden years. And the industry
tried to give the technology its "one last ride" with 157 nanometers.
But in the end, the train had reached its final destination and it was time to move
on. The lithography industry would from then on focus solely on bringing EUV to market,
and the foundries will try to tread water until it arrived.