Roughly every two years, the density of transistors that can be fit onto a silicon chip doubles. This is Moore’s Law. Roughly every five years, the cost to build a factory for making such chips doubles, and the number of companies that can do it halves. 25 years ago, there were about 40 such companies and the cost to build a fab was about $2-4 billion. Today, there are either two or three such companies left (depending on your optimism toward Intel) and the cost to build a fab is in excess of $100 billion. Project these trends forward another ten years and you can expect a single factory to cost nearly half a trillion dollars, and the number of companies that can do it should drop to less than one.
The plot from Gordon Moore’s original paper
Nanometer Numbers
The cutting edge of transistors are “2 nanometer”. Intel has abandoned nanometers as a metric and have begun using terms like 20A, 18A, and 14A, measuring in angstroms (1/10 of a nanometer, or the approximate width of a typical atom). However, these nanometer numbers are entirely fake today. This used to be an objective measure of the width of the gate on a planar transistor, but planar transistors stopped working 15 years ago. Their current substitute, FinFET transistors, have no equivalent feature to measure. The physical feature these numbers once measured no longer exists, but some naming scheme is still needed.
The advertised transistor density of modern 2nm nodes is somewhere around 200-250 million transistors per square mm. There are one trillion square nanometers in a square millimeter, and dividing this out this gives us a footprint of 4000-5000 square nanometers per transistor, or roughly a square 60nm on each side. The gate pitch, or minimum distance between transistors, is generally in the range of 30-40nm, and the 60x60nm footprint is a result of geometric constraints on how densely irregular circuits can be packed, leaving unavoidable empty space.
The extremely tiny features on a chip are produced via photolithography – a light is flashed through a photomask to project an image onto a wafer coated in a light-sensitive photoresist, which hardens or liquifies depending on its exposure to light. The tinier the image can be made while retaining focus and detail, the smaller components can be made.
The hype over the past few years around EUV (Extreme UltraViolet) lithography is mostly in its extremely fine resolution. Previous generations of lithography equipment relied on wavelengths of ultraviolet light measured in hundreds of nanometers, which puts limits on just how small of patterns can be created on a chip. This was cheated somewhat using multipatterning – if your “pixels” are 200nm across, you can shift the image by 100nm and project a new image, giving you some extra resolution in how the two images overlap. You can even do this multiple times, and employ a number of other tricks to cheat out much finer details than should be theoretically possible, at the expense of much more complicated and defect-prone manufacturing. EUV uses light with a wavelength of only 13.5nm, which lands in the overlap between “extreme UV” and “soft X-rays”. Being technically a form of X-ray (prone to passing through things), optical manipulation of this light is difficult – magnification requires atomically-precise curved mirrors relying on the Bragg effect to diffract the X-rays backward, as traditional materials are very ineffective at conventionally reflecting X-rays. X-ray lenses are effectively off the table.
EUV gives us “pixels” of about 13.5nm to build our chip from. ASML is now rolling out its “High-NA EUV” machines, which exploit some optical tricks to get this resolution down to about 6 or 7nm.
These “pixels” are still dozens of atoms across. We could theoretically scale down further – a 6x6x6nm cube has about a quarter million atoms. I suspect that if someone were simply willing to bite the bullet and attempt a 1:1 scale between wafers and photomasks, all this complicated optics could be eliminated and soft X-rays with 1-2nm wavelengths or less could be used. A typical medical X-ray tube is orders of magnitude more energy efficient at turning electricity into photons than the method of lasering molten tin droplets that’s currently used to produce EUV light. An X-ray tube is a simple vacuum tube with an electron beam between a cathode and anode, and the specific wavelength produced is a function of the atomic number of whatever material that the anode is made from.
The next problem however is not the wavelength of the photons, but rather the chemistry of the photoresist. Conventional photoresists are based on polymers – long chains of molecules that link or unlink when struck with photons. Linking them together forms a spaghetti-like knot on top of the silicon. However, the links of these chains are not infinitely small – they are made from atoms just like everything else, and they only work when forming long enough chains. Conventional photoresists stop working below 7-10nm, and most unconventional resists stop working below 5. I’m not aware of any clear plan for scaling past this fast-approaching point.
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