EUV Lithography

The Future of Moore's Law

At the heart of all the research I follow is Moore's Law (the doubling of transistor density on a chip about every two years). While raw computer power alone won't create transhuman AI, large scale brain simulations, or genomics miracles, it provides a fundamental, enabling substrate for those enterprises.

pitch vs gate length

photo credit: Beyond Moore

I wrote this article using a 2012 Intel Ivy Bridge cpu designed at the 22 nm design node.

The design node refers roughly to the size of the smallest features on the chip — usually the gate length. Pitch refers to the distance between successive transistors (like the distance between fence posts, see above) and is usually about four times the node size.)

For comparison, a bacterium is about 1,000 nm (= 1 micron) and a red blood cell is 7500 nm. The width of a human hair is about 70,000 nm. One nm = 10 angstroms = four * the width of a single silicon atom. Each (single) strand of DNA is about 2 nm in width (but centimeters long).

The cpu that followed Intel's Ivy Bridge was their Broadwell cpu that was released in 2014 at the 14 nm node, and in 2016 Intel released its Skylake cpu again at 14 nm.

EUV — which has been perennially delayed — was too late for 14 nm and may even be too late for the 10 nm node, scheduled for release by Intel in 2018.

So, when will Moore's Law hit a brick wall? For CMOS transistors, it may be within a decade — but can it even go that long? Every two years I read the new edition of the ITRS Roadmap on Emerging Technologies. CMOS transistors may eventually be replaced by one of several other promising methods for storing bits and logic, eg graphene or carbon nanotubes (with possible assistance by bottom-up directed self-assembly (DSA.)

Meanwhile, what are the prospects that the semiconductor industry will be able to maintain the every two to three year tick-tock of Moore's Law?

For the past several years all wafer fabrication has used 193 nm light even for my 22 nm Ivy Bridge cpu chip. It's astounding — it's like using a paint brush for a house to do a fine portrait. It's possible, but the many tricks for doing that (water immersion, multiple patterning, fine stepping alignment) are almost played out, although they may make it possible to get to 10 nm.

Moore's Law

A bright ray of hope that may allow Moore's Law to continue to < 10nm is extreme ultraviolet light (EUV). The industry has standardized on 13.5 nm EUV, and it's so difficult to generate and focus that only one key player remains: Cymer, based in San Diego.

In 2012 Cymer was purchased by Netherlands-based ASML, a world leader in photolithography. And, more importantly, to share the risk of this difficult R and D, ASML negotiated a 15% buyout of its own stock by Intel. Taiwan Semi and Samsung also bought large positions.

This Cymer video explains how their LPP (laser-produced plasma) light source works. It takes heroic effort to get enough power as this video on pre-pulse shows. The bottom line is that EUV litho is now working; they have shipped six of their pre-production NXE:3100 machines, and customers have exposed more than 30,000 wafers. Furthermore, their production EUV machine, the NXE:3300B is being shipped and 11 orders have been delivered. EUV volume production was begun (in research mode) earnest in 2014.

Cymer NXE 3300b Vessel

Cymer NXE:3300b Vessel

But note carefully that in 2014 only "critical layers" of wafers were being experimentally imaged using EUV. This is because the power of the EUV source was still too weak to allow real-world high wafer throughput. Here Cymer describes their process for laser generation of 13.5 nm EUV light.

Although the process begins with a 30,000 watt CO2 laser used to explosively create EUV from 30 micron tin droplets, the power of the EUV at the intermediate focus in 2014 was only about 55 watts. And, by the time the EUV is focused by a series of reflecting mirrors only about a hundreth remains to expose the photoresist-coated wafer. (But, CYMER/ASML has now hit around 100 watts of power — so things are looking bright.)

So, the current machines work, but more intense EUV is needed before the industry can commercially produce all layers of 10 nm node wafers with EUV. It's now considered a matter of when, not if. Whether ASML can actually pull this off has been a matter of intense speculation for decades. See these articles by Vivek Bakshi. Meanwhile, Intel with its multibillion dollar investment in ASML is trying to minimize the risk of failure. Intel's Mark Bohr has stated that Intel has a path down to 10 nm even without EUV (by using quadruple patterning with 193i), but Intel hopes that won't be necessary. Quadruple patterning is at best four times as slow as single exposure.

The extreme challenges posed by EUV and by next generation lithography were summarized by IBM's Gary Patton and others at the recent Common Platform Technology Forum (talks are online).

Adjacent to the Stanford campus is a 3 km long linear electron accelerator (SLAC) that has been redesigned as a source of coherent light (LCLS) for imaging of single nm biologic molecules and advanced materials. Another accelerator at Lawrence Berkeley National Lab (LBNL) is being used to create synchrotron radiation and has been used in a collaboration with the semiconductor industry. The Sematech/LBNL collaboration has created tools for testing EUV for wafer fabrication.

Despite the availability of low cost synchrotron radiation (Lyncean), it appears that devices like this may only be used for metrology, eg imaging and defect analysis, rather than for fabrication. Nonetheless, my SLAC physicist neighbor (and fellow juggler,) Gennady Stupakov, suggests that tunable free electron lasers may solve the problem of increasing EUV energy. I follow EUV progress (and challenges) on the website semimd.com . Here's an update that appeared in 2013 — Taiwan Semi was to attempt to use it for 10 nm production in 2016.) (2016 update: come back to this article later this year, and I'll update this.)

Apart from EUV another former bright hope for continuing Moore's Law was e-beam lithography. This is still being pursued by a few companies but to be cost-competitive it requires the parallel use of millions of simultaneous electron beam writers. Without further progress, this is apt to be a niche technology. (But, electron microscopy (SEM / TEM) has been a mainstay of biology research since my youth. (Electron beam writers, however, are apt to be a niche lithography technology.)

Below 10 nm a number of methods for directed self-assembly (DSA) are being explored. DSA refers to the use of block copolymers to create fine patterning between coarser lines printed with lithography as in this video and in this article. DSA is now creating a huge buzz among litho players. But, that's a story for another day.