TWINSCAN: A 20-year innovation in lithography

In 2020, ASML shipped the first dry NXT system in history. This is the first lithography system capable of processing more than 300 wafers per hour - thanks to the TWINSCAN platform on the system.

In the early 1990s, ASML's first PAS 5500 lithography system platform was officially shipped. At the same time, precision equipment and interferometer expert Bert van der Pasch was also conducting research on the interferometer system and wafer transfer module of the PAS platform.

Bert van der Pasch started his career at Philips and has acquired deep knowledge of lithography technology: "I was working on a lithography machine and encountered seven dynamical shortcuts. Many people would have investigated one of them and changed it if necessary. But we didn't. We decided to fix all seven at once."

Bert van der Pasch, ASML researcher and expert in position measurement systems for lithography scanners

Bert led the team to continue to innovate and keep PAS 5500 in the leading position, providing industry-leading productivity and resolution at the time. But with the advent of a new era, ASML realized that a revolutionary innovation was needed to help customers achieve the next stage of productivity.

Like many breakthrough technologies, the solution is simple in retrospect. The wafer must be accurately measured before the pattern is exposed to the wafer. Both measurement and exposure take time, and we have been committed to reducing the time required for each process to improve efficiency. To save more time, why don't we start exposing the previous wafer while measuring one wafer?

“This is a limited innovation, but the revolutionary part of the TWINSCAN architecture is the swapping of the two platforms,” Bert explained. “A lot of this is normal upgrade iteration, but how to swap the two wafer platforms is very different, and we have to make this step work properly.”

Thus, the TWINSCAN platform was born. TWINSCAN is the first and only lithography system with dual wafer working platforms. Wafers are alternately loaded onto the TWINSCAN platform. While the wafer on one platform is being exposed, the other wafer is loaded onto the second platform for alignment and measurement. Then the two platforms swap positions, and the wafer originally on the second platform is exposed while the wafer on the first platform is unloaded. Then, the new wafer is loaded, aligned, and measured.

The first batch of TWINSCAN systems - AT:400 renderings

In 2001, the first TWINSCAN dual wafer platform system using this revolutionary technology was shipped. The TWINSCAN AT:750T is a KrF system that uses a light source with a wavelength of 248 nm to support 130 nm process production. Soon, i-line systems also introduced a dual wafer platform, the TWINSCAN AT:400T; and then an ArF system, the TWINSCAN AT:1100, was introduced to cross the range of currently used lithography technologies, allowing all chip layers to be exposed on the new platform.

As with the earlier PAS 5500 platform, ASML's continued innovation has provided incremental improvements in resolution, overlay accuracy and productivity for the TWINSCAN platform - in the form of platform upgrades, new system upgrades and field upgrades.

In 2004, TWINSCAN made its first improvement with the switch from AT systems to XT systems. The XT systems are 25% smaller than AT systems but have the same or better performance. This enables chip manufacturers to produce more chips in the same size factory.

In the same year, ASML launched the first immersion lithography system: TWINSCAN AT:1150i. Immersion lithography technology increases the numerical aperture (NA) of the lens by adding a layer of ultrapure water between the lens and the wafer - a measure of its ability to collect and focus light. In traditional dry lithography technology, NA can only reach about 0.93, and immersion lithography technology makes it possible to reach NA 1.35.

“Immersion lithography is a revolutionary change,” Bert said. “At first you will doubt whether this can really be achieved.”

ASML engineers celebrate the launch of the first mass-produced immersion system, the TWINSCAN XT :1700i

Higher NA enables lithography systems to provide better resolution and depth of focus with the same wavelength of light, making it easier to manufacture smaller chips. This paved the way for the emergence of modern technologies such as touch screens and Bluetooth, and also made laptops more popular than traditional personal computers.

Around 2000, the immersion ArF systems used by chipmakers approached the resolution limit of conventional photolithography. However, chipmakers’ need to shrink chip size did not disappear.

During this period, the industry began to explore a method called "double exposure" (also known as "multiple exposure"), which is to split a complex layer pattern into two (or more) simpler patterns that can be exposed separately to produce the original pattern.

As the name implies, double patterning requires twice as many exposure steps and the ability to align the two patterns extremely closely. To make it cost-effective and feasible, lithography systems must become faster and more precise. This has led to another major improvement in TWINSCAN: the NXT platform, a completely redesigned and significantly lighter wafer platform.

Just like when it was busy developing PAS 5500, facing the pressure of customer production needs, ASML chose a revolutionary approach.

The first NXT system, the TWINSCAN NXT:1950i, was launched in 2008, increasing productivity by 30% to more than 200 wafers per hour while also improving overlay accuracy to 2.5 nanometers. Today, the leading NXT immersion system can process 295 wafers per hour with an overlay accuracy of 1 nanometer.

Historically, big leaps in lithography system resolution have come from changing the wavelength of light used. Immersion lithography and multiple-pattern lithography changed that trend for a time, allowing chipmakers to keep their product roadmaps open for smaller and more advanced chips while transitioning to many new technology nodes using familiar ArF lithography techniques.

As we all know, innovations in lithography technology tend to focus on cutting-edge areas, creating new systems for key chip layers that support smaller line width dimensions.

In fact, critical layers and cutting-edge lithography systems account for only one or two of the more than one hundred layers that make up a modern chip. Most of the layers of chip manufacturing use mature lithography technology to expose larger line width dimensions. For these layers, speed is more critical, and the fastest lithography systems use the TWINSCAN NXT platform.

As immersion NXT systems are increasingly used for sub-critical layers, ASML decided to expand the NXT platform with "dry" technology for sub-critical layers. As a result, TWINSCAN NXT:1470, the first dry system to achieve overlay accuracy better than 4.5 nanometers on a product, became the first lithography machine capable of processing more than 300 wafers per hour.

The first TWINSCAN system is shipped

The TWINSCAN dual-wafer platform has celebrated its 20th anniversary of commercial use, and its achievements are obvious to all, continuing to help solve various challenges in the development of the semiconductor industry. In 2020, we celebrated the 100th refurbished TWINSCAN, which is also ASML's commitment to the continued development of the circular economy. As an industry-leading lithography platform, the continuous development of TWINSCAN will continue to help chip manufacturers improve performance and reduce the overall cost of chip production.

"The market expects us to perform better and better and produce better and better products," said Bert. "Thankfully, our employees are the same as they were in the 1990s - always energetic and committed to making some changes, optimizing it and making it work. This is the culture of ASML and the starting point for every one of our employees to be proud of working at ASML."