Vivek Bakshi, EUV Litho, Inc.

Author’s preface: This article is a departure from my usual high-tech language, because I think our industry needs to do more to educate non-technical readers about how their treasured electronic devices got to be so cheap and powerful. Our success in realizing Moore’s Law has been one of the greatest achievements in modern science, and we must continue doing all we can to continue that progress. Please feel free to share this essay as a holiday gift to anyone in your life who benefits from the achievements of lithography.

For many, our Christmas holiday cheer is wrapped around getting the latest gadget that brings us more power to do things than we had the year before ‒ be it a new iPhone, iPad, laptop or some other high-tech gizmo. Added to this annual ritual, which we now intuitively expect but do not quite notice, is that we pay less or the same for these gadgets than we did in previous years – even though they may run twice as fast and store three times as many photos and videos. At the heart of this happy surge are the computer chips that grow more powerful every year without increasing their price. I would like to tell you how we in the computer chip industry do this, and what it will take to continue this trend in the coming decades.

Technology was not always like this. Growing up in the early 80s in India, where my dad worked for the telephone company, I remember that when we got a new phone it was the same rotary dial model with just a new exterior body, and maybe a new color. If today’s technology were moving at the same speed as then, we would only be getting a new cover for our iPhone or a new computer mouse for Christmas, and not more powerful gadgets.

To understand this phenomenon, we need to look at the leading-edge computer chips that are the heart of all these tools, made by leading chipmakers like Intel, Samsung and others. Inside these microchips are tiny transistors and other circuit elements that do the work. The reason these devices can deliver more power every year at lower cost is because the advancement of computer technology is guided by Moore’s Law, named after Gordon Moore, co-founder of Intel Corporation. Moore proposed this law in 1965, saying that number of transistors per square inch would double every two years or so.

We have been able to follow Moore’s Law so far by making transistors and other circuit elements smaller every year. Making computer chips takes many steps, the most critical of which are embodied in a process called Lithography, which involves printing the images of circuits. To print smaller and smaller transistors, we need to be able to resolve the printed images. British physicist Lord Rayleigh (1842-1919) pointed us to “knobs” that we can turn to resolve ever-smaller images. Prominent knobs are color of the light for printing (wavelength), design of optics (numerical aperture) and printing under something more dense, like water. We also have also learned lots of tricks (called optical proximity corrections and multiple patterning) that let us keep on printing smaller and smaller features.

The current technology of choice for advanced printing of computer chips is called 193 nm optical projection lithography, which involves a zillion optical tricks and repeats the printing process three or four times to make one image. However, 193 nm has been running out of steam for some time. This means that either we cannot make computer chips more powerful by just shrinking the size of features, or the cost of doing so will be a lot more. Neither of these are acceptable solutions, and that is where Extreme Ultraviolet Lithography (EUVL) comes into play.

EUVL promises to extend Moore’s Law by changing the color of light used for printing – from current 193 nm light from excimer lasers to 13.5 nm light from plasma sources. Alas, we cannot see either wavelength with our unaided eyes. This switch of color came with big physics challenges as EUV Light, with its photons of 14x energy, interacts with matter very differently than photons from excimer light. This change has resulted in a massive amount of work over many decades on light sources, optics and photo-sensitive chemicals for developing images. For these reasons, EUVL has taken many decades of worldwide effort and investment and is now expected to be used by leading chipmakers by 2018- 2020 time frame.

We would certainly be lost without the ever-more powerful computer chips that we are now used to having at our disposal every year. So now you know whom to thank for your new holiday gadgets, and you can rest assured that they will keep on working to ensure the benefits of Moore’s Law will continue for years to come.

By Vivek Bakshi, EUV Litho, Inc.

The 2016 Source Workshop was held Nov 7-9, 2016 at ARCNL, Amsterdam, The Netherlands. During the workshop we received new information about EUV source power, updating what we learned at the EUVL Workshop in June. ASML now has 125 W sources in field with their uptime improving, and 210 W dose controlled sources in lab, with 5.5 % conversion efficiency (CE). This leads me to predict that we will be able to have 250 W in field by 2018, which will be needed to support manufacturing at 125 wafers per hour. Another piece of good news on the high power source front is the continued solid progress by the second-largest supplier of HVM EUV sources, Gigaphoton. They now have 100 W @ 5% CE, with 95% duty cycle for 5 hours of continuous operation.

There were reports of continued progress on EUV metrology sources, but they are still years away from being integrated into the next generation of mask defect inspection tools. In the workshop, I heard that Zeiss is now working closely with suppliers to evaluate their EUV metrology sources for their next generation AIMS tool. We also need patterned mask inspection (PMI) tools to be ready sooner than later, and I was happy to see a presentation by KT on the status of the source for their PMI tool. However, when and if this tool will become reality is still unknown, while these tools will be needed at 5 nm application of EUVL in fabs.

This year there were several papers (experimental and theoretical) on how to increase CE of sources by looking deeper into the working of EUV sources. In EUV sources, the laser energy (which is at 10 micron wavelength) is converted into 13.5 nm photon. Current reported CE is 5.5 to 6%. Can we can get more efficient?

We learned, via plasma measurements, how we can better tweak the delay and shape of laser pre-pulses (Kyushu University papers) and were told about development of pre-pulse lasers to enable the delivery of those optimum pulses (work from HiLase).

I found interesting the work of Hanneke Gelderblom, Univ. of Twente and Dmitry Kurilovich, ARCNL. They are using “water drops as scale model for tin” to understand the scalability of hydrodynamic stability of droplets interacting with lasers. They found that they can adjust parameters to work in regions to avoid drop breakups during interaction of laser with droplets, while looking for greater laser absorption to increase CE. It was a good example of how we can use learnings from other disciplines to improve the functionality of EUV sources.

Gerry O’Sullivan of UCD pointed to the need for maximizing the line emission by reducing opacity and reducing recombination. He noted that plasma density has a “sweet spot” for a maximum CE and optimized CE. He also described his wedged target colliding plasma that can be better matched to CO2 for increasing CE.

A most interesting CE paper to me was one by Mikhail Basko. He pointed out that in principal, 20% CE is possible (based on 40% spectral efficiency calculations) but in reality only 9% CE can be achieved. He pointed that 2.5% of CE is lost as the kinetic energy of plasma flow, while rest of CE is dissipated due to non-uniformity of temperature across the “working” zone and in-band reabsorption. We need to find ways to achieve this optimum density profile in our tin targets to get to 9%.

There were several interesting papers on modeling efforts to improve CE (LLNL, ISAN, Cymer) and generation of fundamental data to improve modeling. Such efforts are going to be important as we run out of knobs readily available to us today to improve CE, and we must look deeper into the working of plasma sources to squeeze those additional EUV photons out of plasma and search for stable operational modes for sources that can be sustained in factories around the clock.

We had many excellent papers on XUV sources and their applications to support manufacturing in the semiconductor industry and beyond. Hans Hertz in his keynote speech described his water window microscope, which with a 200 W laser of 600 picosecond pulse operating at 2 kHz gives an early synchrotron level of brightness. It can now do 3D tomography with a 10s exposure. These developments were possible due to a new multilayer mirror with >4% reflectivity (optiXfab) at water window wavelengths. He had reported the development of these new multilayer mirrors in last year’s source workshop, and decided to incorporate them in his tool to achieve this progress.

This year’s workshop had the highest attendance ever. I was happy to see continued work by the research community and suppliers to better understand the working of EUV sources, so that we can achieve those 500+ W sources that can operate 24/7 in fabs with 80-90% uptime.