Rare earth elements are the vitamins of the economy.
We don't need a lot of them, but we die if we lose them.
There are 18 rare earths - the 15 lanthanides plus yttrium, scandium, and lutetium.
I also hear 17 but whatever.
Their single biggest use case are for magnets - namely the powerful permanent magnets for
items like electric motors and generators.
But as I mentioned these elements randomly pop up in a whole smorgasbord of different
industries.
Could one of those industries be the semiconductor industry?
I have been curious about this one for a while.
Does silicon have a rare earths problem?
In this video, we are going to look at semiconductors and the rare earths.
## Reintroduction
I did a video about this topic a while ago.
In it, I mentioned that rare earths are not actually that rare.
For instance, the most common rare earth element cerium is more common than copper and three
times more common than lead.
The big issue is less about actual rarity as it is refinement.
They are never found by themselves in nature and so have to be extracted and produced using
expensive, toxic processes.
The United States pioneered the use of these rare earths, and still runs a mine out there.
But today China dominates the space.
Not only in extraction, but more critically in refinement.
If you want to learn about all that, I highly recommend watching the other video.
Done with review.
Let's get into the first rare earths application.
## Lasers
Let us start with lasers.
Semiconductor manufacturing is highly dependent on lasers.
A laser has a few components.
We have a gas or element used to create the laser light - the "laser medium" or "gain
medium".
We stimulate the medium's atoms into producing laser light using an energy source like a
krypton lamp or LED.
The medium releases light in all directions so we use mirrors to amplify and focus that
light to create the laser.
We can use a wide range of materials to produce laser light.
But rare earth elements - particularly the lanthanides - are particularly ideal because
they emit a very precise wavelength of light.
The most widespread laser is the Neodymium-doped yttrium aluminum garnet laser - or YAG lasers
as I’ll call it.
We use this solid state laser a lot for medical applications - laser eye treatment or dentistry
- but it is commonly used in semiconductors.
## Laser Lithography
Lasers help produce the EUV light for our advanced lithography machines.
We initially used ND:YAG lasers to research the EUV light generation mechanism.
But later, ASML switched to carbon dioxide lasers for debris reasons.
Though that does not mean we are now free of the rare earth curse.
During operations, the carbon dioxide in the carbon dioxide laser turns into carbon monoxide,
which poisons both us and the laser.
So we use a rare earth oxide - lanthanum strontium cobalt oxide - to catalyze the carbon monoxide
back into carbon dioxide and maintain the laser's operation.
This is just one example of the small but critical roles that rare earths play in modern
technology.
## Laser and Cleaning
Another prominent laser use case is for the dry-cleaning of wafers.
Metallic contaminants and foreign particles are common.
Recall that washing makes up 30-40% of the steps in the semiconductor fabrication process.
We frequently use water and chemicals like hydrochloric acid for this - something known
as "wet cleaning".
But as particles get smaller, we have to overcome new, very powerful forces.
A particle less than a few microns wide adheres to the wafer using Van Der Waal forces - a
force which grows in proportion to the particle's diameter.
Or D.
These forces must be overcome by the force of the clean in order for that clean to dislodge
the particle.
But the clean's force can only be proportional to the square or cube of the particle's diameter.
Depending on which force you are talking about, gravity removal or drag.
It is like a grip.
We have a better grip on large things than small things.
So anyway.
If we turn this into a math equation, the ratio between the adhesion ratio - D - and
the removal force - D squared, or D cubed.
We can simplify this to 1 to D or 1 to D squared.
This ratio gets bigger as the D shrinks, implying that we need much more force to remove the
particle.
Things can indeed get very difficult for those dealing with small D's.
But lasers can help.
There are several principles behind laser wafer cleaning - depending on how you do it.
In one method, we directly blast the substrate underneath the particle with a laser.
This transfers thermal energy from the laser into the substrate, causing the substrate
to very rapidly expand for a brief period.
This pushes up the particle enough to overcome the Van Der Waal forces, spraying it upwards.
A second methodology follows a similar principle.
You deposit liquid onto the substrate and then very quickly blast that liquid with the
laser.
The liquid evaporates and that pushes the particle up off the substrate.
The benefits of laser cleaning is that it is effective and efficient.
At the same time, we limit our use of toxic and potentially dangerous chemicals like acids.
It is likely used in fabs and represents a significant use case of rare earth minerals.
## Permanent Magnets
Permanent magnets are most critically used in the new energy space.
But they have significant applications within semiconductor manufacturing too.
For instance, there are some interesting precision motion systems based on magnetic levitation
for photolithography machines.
But the one very significant use case for magnetics - via permanent magnets or electromagnets
- is in the world of plasma material processing.
A plasma is a highly ionized gas produced by exposing gas to electric or magnetic fields
which accelerate their electrons.
Plasma material processing is the science of manipulating plasma - which feels like
a power an X-man should have.
The industry most prominently uses plasmas for thin film growth and patterning.
With it, we can evenly deposit a high quality film of material - usually a metal like aluminum
but also silicon dioxide - onto the wafer for etching later.
Tokyo Electron and Applied Materials both provide a version of this plasma-enhanced
chemical vapor deposition system.
To uniformly control the plasma and keep it effective, we deploy a powerful magnetic field
across the wafer.
The primary system for doing so is almost always an electromagnet, but auxiliary magnets
might use rare earth permanent magnets.
## Rare Earth Coatings and Abrasives
One of the smaller but ever-present uses for rare earths is for coatings and abrasives.
Tracking down these things are supply chain nightmares.
We use a yttrium oxide-based coating for plasma etch chambers, where they help cut down on
maintenance and cleaning costs.
I have seen the semiconductor companies in China use cerium dioxide abrasives as part
of the chemical-mechanical polishing or planarization process.
Shortened down to CMP.
CMP is for polishing the wafer down to an extremely flat surface, making it more suitable
for photolithography.
It is called chemical-mechanical because it has both a chemical aspect - a slurry with
the aforementioned abrasive - as well as a mechanical aspect - a polishing pad.
## High-K Rare Earths
So rare earth elements are often used within the semiconductor equipment.
But how about the transistor itself?
I have maybe one use case.
One of the big challenges with the shrinking size of the transistor is how to prevent electrons
from leaking through the gate as they pass from the source to the drain.
Such a thing causes power drain for the user.
In 2007, Intel introduced a new gate - the High-K/metal gate.
The "High-K" in the name refers to the fact that the insulating element under the gate
has a dielectric constant higher than silicon dioxide, the traditional gate material.
This cuts down on electron leakage.
Intel's original High-K gate back in 2007 was made from Hafnium Oxide, which has a dielectric
constant of 20-25 as opposed to Silicon Dioxide's 3.9.
But there are two major problems with the Hafnium oxide.
First, its dielectric constant still could be higher.
To quote an HBO series, 20 is not great, not terrible.
Second and more significantly, Hafnium Oxide has manufacturability issues.
We cannot easily deposit it onto the substrate as it has problems bonding with silicon.
And it crystallizes at low temperatures - about 400-500 degrees Celsius.
So when we try to deposit it onto the substrate, the resulting Hafnium Oxide layer is uneven,
not quite crystalline, and somewhat defect-prone.
Now Hafnium itself is not a rare earth element, but they are similar.
So this triggered a lot of research in the rare earths space for an alternative material.
Rare earth oxides can bond better with silicon and present an intriguing alternative.
I am aware of continued research of High-K rare earth oxides in China as recently as
2020.
Perhaps searching for a game-breaking rare earth advantage in semiconductors.
However with the advent of 3D transistor structures like the FinFET, the rest of the semiconductor
industry - Intel included - has largely moved on from High-K dielectric gates.
## Rare Earths in LEDs
Rare earth ions are widely used in lasers because of their luminescent properties.
These properties give them a lot of importance in the world of optoelectronics - namely white
color LEDs.
Today, one of the major ways we produce that white light is through the use of color-converting
phosphors.
We need these phosphors because LEDs emit light in a certain range or color.
White light on the other hand is made up of all the colors.
Phosphors are preferred because of their luminescence efficiency, heat resistance, and chemical
stability.
Here is how it works.
So we have a blue LED emitting blue light.
Also within the LED package we have these phosphors - usually as a coating.
They absorb the blue light and in turn emit yellow, green, or red light - depending on
whatever they are made from.
That all combines to create white light.
Those color-converting phosphors include rare earth elements.
For instance, the yellow phosphor is made from Cerium-doped yttrium aluminum garnet
micro-powder.
Other used elements include europium and terbium.
This is a pretty big global market - worth about $76 billion USD in 2020.
White LEDs are increasingly being used everywhere we have artificial lighting - including display
devices, imaging systems, monitors, therapeutics, and so on.
Losing access to these rare earth materials can be damaging to the LED supply chain.
Cerium is pretty common and commonly used in industry so we can still source it from
elsewhere.
But we don't have much yttrium.
It represents just 0.12% of the rare earth elements in America's only rare earth mine,
California's Mountain Pass Mine.
47% of the world's yttrium production is used for white light LEDs.
It is a significant component in these LEDs too - we use 300 milligrams of it for your
typical 12 watt white light LED.
Alternatives are difficult.
We haven't yet found something with the combination of chemical stability, thermal stability and
light efficiency.
Furthermore, many compelling alternatives - like nanocrystal quantum dots - are made
from toxic materials.
## Silicon Photonics
Adjacent to the world of optoelectronics is the world of silicon photonics.
Silicon photonics is being studied as a way to transmit information, create better LiDAR
or even replace the interconnects of a neural network chip.
I did a video about the topic earlier.
However, one of the big issues with the silicon in silicon photonics is that silicon cannot
emit light by itself.
Generally, this has forced us to produce our light from outside of the chip or use a different
material like gallium nitride.
But silicon can emit light if we dope it with erbium, a rare earth element primarily found
in China.
We can achieve this doping via a process called ion implantation or by layering it on using
an atomic method like epitaxy.
Erbium-based silicon light sources are an interesting technology because they may be
more compatible with traditional CMOS manufacturing processes.
Such a thing has the potential to bring us the dream of a monolithic silicon photonics
chip.
Though right now the market remains a niche in the larger semiconductor space.
## Biomedical
Before I move on to the conclusion, something that stuck me as I researched this was how
rare earths seem arguably more valuable to advanced medicine than for semiconductors.
I previously did a video about scintillator crystals for PET scans.
These crystals inside the scanners receive the positrons emitted by an injected radioactive
tracer and turn them into visible light for processing.
The latest, most advanced crystals for today's PET scanners are Lutetium Oxyorthosilicate,
doped with Cerium and Lutetium Yttrium Orthosilicate, doped with Cerium.
Cerium, Lutetium, and Yttrium are all rare earth elements.
Virtually anything related to bioimaging and light is going to have some involvement with
a rare earth element.
Contrast agents for CT and MRI scans.
Imaging solutions for diagnosing and tracking cancers.
The list goes on and on.
## Conclusion
Ultimately, my evaluation is that rare earths are not significantly present in semiconductor
manufacturing.
As in, we are not directly using them in computing chips themselves like as we are for Neodymium,
Cerium, and Samarium permanent magnets.
However, rare earth elements like neodymium are significantly present in equipment used
for the semiconductor production process.
Most notably, the lasers and plasma material handling.
There are also significant adjacent areas like optoelectronics and silicon photonics
which are very dependent on rare earth elements.
I am especially worried about yttrium and the white light LED.
I am also concerned about not being able to dig deep into these companies' supply chains
for rare earth reliance.
It is possible we might not ever know of these reliances until we lose them.
Who could have known that we use "lanthanum strontium cobalt oxide" for cleaning carbon
dioxide lasers?
America and Europe are willing to throw $100 billion into the domestic production of semiconductors.
Perhaps they can spare maybe $10 billion into something arguably just as vital to the supply
chain?
Some food for thought.