It has been a week and thanks to tests by a few Chinese scientists,
we can probably say that LK-99 isn't a room temperature superconductor.
The hype has largely died down. LK-99 has vanished off people's social feeds and science news.
I suppose we can still keep on waiting but the right now it's so over.
Oh well, it wouldn't have done anything for the semiconductor industry anyway.
I did some follow-up work since the last video.
Even if we hit the jackpot tomorrow and find a True Blue room temperature superconductor,
it will be nice but hardly a revolution. In this video,
some irresponsible thoughts about superconductors and semiconductors.
## Interconnects
The first thing I thought about when I thought
about superconductors for chips were interconnects.
In an integrated circuit, interconnects are wires for
transmitting the electrical signals between the transistors and other circuit elements.
These interconnects are usually but not always made from low-resistance metal,
so we call them "metal layers".
The wires are surrounded by dielectric material - "intermetal dielectric layers"
that serve as insulators. For a long time, we used trusty silicon dioxide for this.
These layers sit all jumbled about on top of the
silicon transistors like how stratum look like in geology.
Generally speaking, an IC has two types of interconnects - local and global.
Local interconnects occupy the chip's lowest levels and connect local circuit
blocks. The wires are very short so designers are less concerned about
line resistance than they are about other factors like heat.
So I don't think a room-temperature superconductor - which would be quite
sensitive to heat - makes sense here at all.
Global interconnects are higher up in the stratum. They connect distant parts of the chip
- delivering important signals like synchronizing clock signals or power to run the transistors.
For this reason, they have great size. They look thick. Solid. Tight.
The distance they have to span also means that they
must be made from low-resistance metals like aluminum or copper.
## Superconductive Interconnects?
I was very curious about this.
In today's most advanced AI accelerators - like the Nvidia H100s we use for massive models like
ChatGPT - some 80-90% of the power budget is spent on simply transferring data around rather than
actual computation. Will superconducting interconnects simply wipe that out?
So I asked MIT PhD Alex Sludds for a few thoughts. He is a photonics researcher with a background in
semiconductor devices and interconnects. And of course, he is a good friend of the channel.
I did a video about his work in neural networks and photonics a while ago. Alex said:
> At short lengths of wire, having a superconducting wire
will help significantly decrease the energy consumption to transmit a zero or a one.
When he mentions "short", he means as in the context of wires on the chip or
between multiple chips like in a chiplet scenario.
> However, this energy will not become free,
since wires will still have a capacitance and an inductance
that stores in the electrical/magnetic field of the current flowing through the superconductor.
What I think Alex is saying - if I get it wrong it's because I'm too dumb to understand his
words - is that even if the interconnects suddenly possess superconducting superpowers, its electric
current will still interact with its surrounding environment in a way to cause it to leak power.
When you push a current through a wire, that wire creates an electric field around itself. Some of
the current's energy gets stored in that electric field, causing loss. This is "capacitance".
The wire will also produce a magnetic field,
and that magnetic field will store some of the current's energy too. This is "inductance".
So in terms of energy consumption, there’s going to be some improvement. That will especially be
the case for global interconnects for transmitting power rather than communication signals. But
the power consumption gains might not be as dramatic as I might have initially thought.
## Speed
What about computing speed?
A substantial portion of the computer's cycle time is spent waiting for data to arrive. So,
if the interconnects can carry the data around faster then that's great. But
there are fundamental limits on how fast we can make this.
The absolute ceiling in signal speed - the fastest that anything can possibly
travel through any medium - is the speed of light. And unfortunately,
signals cannot travel at light's full speed because the wire is not a vacuum.
Additionally, we need to consider a form of interconnect delay known as
"Resistance-Capacitance" Delay or RC Delay. Like as I mentioned earlier,
the interconnects store charge. These delay the electric signal's transmission to the receiver.
Capacitance depends on the dielectric material surrounding the wire. That material has a
dielectric constant, called k-value. It has little to do with superconductors. So even if
we cut the line resistance to zero, we still have to deal with the capacitance part of the
delay equation. I wonder if the speed gains will be as significant as I initially thought.
## Current Density
There is one last thing about the interconnect thing that I want to mention.
Remember in the first superconducting video when Kamerlingh-Onnes - the
discoverer of superconductivity - tried testing superconducting lead wire to generate electricity?
He discovered that superconductivity requires not only a low enough temperature but also a
low enough magnetic field and a low enough current density.
After the discovery of High Temperature Cuprate superconductors, people
investigated using them for interconnects in superconducting integrated circuits.
Such superconducting interconnects - when matched with the right circuits - can
indeed get very fast. The concern however is the current density ...
Which is defined as the electric current divided by the superconductor's cross-sectional area.
As it turns out, the cuprates' critical current density limits are not very high. So in order
to run integrated circuit operations and stay under those limits, the interconnects have to
be very wide. Perhaps even a centimeter wide, which for most digital circuits is unfeasible.
This is why the cuprates never made headway into the integrated circuit arena. LK-99 or
any room temperature superconductor may suffer the same issues. If so,
then it is just a scientific curiosity and copper retains its dominance.
## Manufacturability
Even if the new superconductor somehow miraculously fits into the current IC "box",
we have the manufacturability concerns.
I asked Dylan Patel - chief Analyst at SemiAnalysis - to say a few thoughts. He said:
> While LK-99 seems to show some promise as either an extremely low resistivity material
or in fact even potentially a superconductor the manufacturability of it is still in question.
> While it does seem to be a fairly simple process for a low purity,
low volume amount, scaling this at high volume at high purity with less
than 1 part per billion defect is going to be a tremendous challenge
Dylan has a very good point. TSMC has said that a single integrated circuit
can use over 100 kilometers of copper wiring. They produce billions of chips every month.
To me, this is the biggest issue. Any room temperature superconductor will be a complex
alloy and those tend to not scale. How can we integrate it into the existing process flow?
It took 15 years for IBM to crack the complicated
materials engineering issues surrounding copper interconnects.
And that's copper. We have been playing around with copper for
10,000 years. Any room temperature superconductor will have been known for far shorter.
## Josephson Effect
In 1962, the British physicist Brian Josephson - then a grad student just 22
years old at Cambridge - wrote a paper predicting a phenomena that we now call the Josephson Effect.
The effect occurs when you put two superconductors next to one another with a certain thin insulating
barrier in between them. Very thin, as in just a few atomic layers thick.
The Josephson Effect produces what is called a "supercurrent", where a direct current flows
through from one side of the device to the other despite the physical presence of an insulator.
The supercurrent flows without resistance and with little energy loss. This is in contrast
with traditional transistor gates, which generate heat as they switch. That heat must be dissipated.
The Josephson Effect is one of the few examples
of quantum mechanics - quantum tunneling - that we can observe. So that is pretty cool.
Josephson later won a Nobel Prize for this. Man, they just handing out Nobels for superconductors!
No really, they are. Superconductivity research has won a tenth of all the
physics Nobel prizes awarded since its discovery in 1911.
## Josephson-junction Electronics
A year after Josephson's thesis came out,
P.W. Anderson and J. Rowell of Bell Labs produced an experimental circuit
based on this theory, creating the Josephson Junction Switch.
There are a variety of implementations. The most popular uses niobium as the
superconductor and aluminum oxide as the insulator, creating a sandwich structure.
They can switch on or off in just a few picoseconds, or a trillionth
of a second. We take advantage of this by encoding the data into very rapid pulses,
allowing processing speeds of 100 Gigahertz or higher.
I don't think anyone disagrees that these can get very fast. But there
are major issues. Josephson Junctions require big compromises and suffer
shortcomings in certain aspects. Let me go through a few of them.
## Memory & Complexity
IBM worked on producing Josephson Junction-based computers for two decades,
eventually closing their efforts in 1983.
One of the major reasons why they finally ended the project was that they struggled with producing
a proper superconducting memory circuit. Memory is basic and essential to a computer.
The largest Josephson-only memory unit ever produced was a 4-kilobit unit
produced in 1990 by NEC. The reason the memory issue exists is that it
is extremely difficult to produce these junctions in large densities.
Similarly, this is why the most successful Josephson-based systems have been analog
systems and the like. Analog systems handle real world signals like power
or sound or the like. They have only a few components so it’s easier for
superconductor-based integrated circuits to compete with them.
Sensors too, Alex pointed out that we can use superconducting circuits to produce
some of the most sensitive photon detectors - superconducting nanowire single photon detectors.
Digital systems on the other hand are built with millions and billions of consistent
transistors. It will be far harder to produce a competitive superconducting digital system.
Something that might make the most sense would be like a semiconductor-superconductor hybrid module,
like AMD's chiplets. This is where we combine
semiconductor-based memory systems with superconductor-based processing systems.
## Freezing Concerns
So even if we get a True Blue, Real Deal room temperature superconductor.
And it fits in the IC critical limits box. And it can be manufactured in sufficient purities.
And we figure out how to make dense bunches of Josephson junctions with it. And maybe we do
that hybrid semiconductor-superconductor modules we talked about to overcome the memory problem ...
it still doesn't mean that we get to throw off the freezing equipment like Prometheus's chains.
As a superconductor approaches its upper limit transition temperature,
the number of Cooper pairs in the superconductor declines.
If you recall, Cooper pairs are essential to making superconductivity work.
The less Cooper pairs we have, the more unstable the circuits will be
in the real world. Rule of thumb per BCS theory is that we want to
keep the system's operating temperature at half of its transition temperature.
That is why existing superconducting Josephson circuits made from Niobium are
kept at 4-5 Kelvin despite Niobium's transition temperature being 9.3 K.
LK-99's claimed transition temperature is around 400 Kelvin. So its practical
working temperature would be around 200 Kelvin.
Or around -99.67 degrees Fahrenheit, or -73 degrees Celsius, or 360 degrees Rankine,
or -9.055 degrees Wedgwood, or 179 degrees Leiden
If we want a true blue room-temperature superconducting chip without the freezing
equipment, we need to find something more like an oven-temperature superconductor.
With a transition temperature of 600 Kelvin or 620 degrees Fahrenheit.
Refrigeration remains the single biggest limiting factor in superconductor-based
computers - adding a big energy usage cost on top of the system.
The dream that we all have - the tech-optimist’s fantasy - is that superconductivity at room
temperature is now possible once we get the room temperature superconductor.
I was heartbroken when I learned that it wasn’t so.
## Conclusion
If LK-99 is the real deal, it will be a historic scientific landmark.
It might also do impressive things for the power transmission and energy industries in the real
world. Though I am just saying that off the top of my head without having done the research.
But superconductors aren’t serious candidates for disrupting existing semiconductor technologies. At
the risk of being made an idiot later, I am saying that I don’t believe that any superconductor-based
computer will become commercially competitive at any time in the future.
But hey, the hype was a blast for about a week or so.