Photons as a Service

In January, I wrote about the worsening cost curve of EUV lithography and two startups trying to bend it:

I teased xLight and Substrate and promised deeper dives on both.

Today I’m delivering the first one on xLight. Now that we understand the broader economic problem the industry needs to solve, let’s dive into technical details and areas for improvement.

We’ll start with some fundamental limits of LPP EUV scanners.

The Photon Problem

Every ASML EUV scanner generates its light the same way. It’s called Laser Produced Plasma, or LPP. It’s the “shoot tin droplets and hit ‘em each twice with a laser” light source magic trick. I’m assuming most readers are somewhat familiar, but if not here are great YouTubes references get you up to speed plus a short recap

How EUV’s light source LPP works: A high-intensity CO2 laser fires at tiny droplets of molten tin traveling at 100 meters per second, 50,000 times per second. Each droplet is actually hit twice: first a pre-pulse reshapes the 30-micrometer tin sphere into a concave sheet, then the main CO2 pulse vaporizes it into a dense plasma that emits 13.5nm EUV photons. These photons bounce off a series of collector mirrors, pass through a photomask, and expose the wafer below. As you can imagine, it took decades to get this concept to work at reliability and yield needed for scaled manufacturing. It’s freaking nuts that it works. That’s what I love about the semiconductor industry by the way, literally everywhere you look it’s just mind-blowing technology.

One interesting thing to note is that the industry actually first tried particle accelerators called synchrotrons to generate this high-energy, small-wavelength light. But at the time, synchrotrons didn’t have enough directional light. Next they tried xenon gas lasers, but that had a fundamental physics ceiling resulting in less than 1% conversion efficiency. Eventually the industry landed on tin back in 2002. Blasting tin particles into a plasma produces EUV light, but it has a debris problem (contaminating everything). That’s an engineering problem though (how to deal with this debris) but not a fundamental physics roadblock.

Fast forward 20 years and LPP with tin works. Hitting 50,000 falling tin droplets per second with a laser, twice each, 24/7. It’s genuinely an engineering marvel, should be discussed in freshman physics and engineering classes to get people excited about science

But.... the best LPP achieves only about ...