ASML DUV Lithography Detailed Science
According to ASML official science, starting from the light source, along the optical path including lighting modules, projection objective modules, and immersion lithography, in addition to the lithography machine, there is another “god”, which gathers the most important moving parts in the lithography machine, is the mechanical system ” heart” of the system, it is the wafer stage module.
As the most critical equipment in the chip production process of photolithography has a very high technical barrier. In such a time-critical industry as a chip, time is money. According to the official introduction of ASML, ASML has been pursuing the ultimate speed of lithography, the most advanced ASML DUV lithography machine can complete the production of 300 wafers per hour.
In other words, it takes only 12 seconds to complete a whole wafer, which also deducts the wafer exchange and positioning time, the actual lithography time to be shorter. And a wafer lithography process, the need for nearly 100 different locations on the wafer imaging circuit patterns, so the completion of 1 image cell (Field) exposure imaging is also about 0.1 seconds.
So the animation you see is actually slow motion. To achieve this imaging speed, the wafer platform is moving at a high acceleration of up to 7g. What is the concept of 7g acceleration? The F1 racing car from 0 to 100km/h acceleration takes about 2.5 seconds, while the acceleration of the wafer platform of 7g, while the acceleration from 0 to 100km/h as long as about 0.4 seconds.
The accuracy of the lithography is including the horizontal direction and vertical direction. In the horizontal direction, the chip manufacturing is stacked layer by layer upwards, up to hundreds of times stacked.
Each stacking must be perfectly overlapped with the previous one, the overlap error, called overlay precision (overlay), now the requirements have been to 1 ~ 2 nanometers. The wafer is transferred from the transfer module and placed on the wafer platform, there will be a certain mechanical error, and the error of precision machinery is micron level (1 micron = 1,000 nm), which means that each wafer on the wafer platform, there is always a shift of more than a few thousand nanometers.
How can ASML achieve 1~2 nm accuracy for each stacked lithography?
From the vertical plane, because the projection objective of the lithography is too huge, the acceptable clear image range above and below the focusing point is less than 100 nm. And from the microscopic point of view, the wafer surface is uneven, and if the height difference of the wafer platform is accumulated, the height of the photoresist at different locations on the wafer surface can vary from 500 to 1,000 nm.
These huge offsets and height differences make it necessary to do precision measurements for each wafer before each exposure, intercepting the tiny nanometer-level errors in each block of the wafer. Real-time corrections are made during the exposure phase to achieve nanometer-level accuracy.
The dual wafer platform is also developed by ASML lithography to achieve both speed and accuracy. Accurate measurement is indispensable but takes a lot of time. Dual wafer stages allow one wafer stage to be exposed to the wafer while the other stage can be measured and corrected for the next wafer. Achieving a seamless transition between measurement and exposure greatly improves production efficiency.
After ensuring speed and accuracy, the stability of the machine has to be considered.
The ASML lithography machine uses the so-called balance mass to absorb the reaction force exerted on the machine base by the balanced wafer platform, which is perfectly balanced. The entire machine is completely stationary and stable.
The lithography machine performs scanning exposure (scan) with extremely high acceleration. In less than 0.1 seconds, it has to stop and scan in the opposite direction. If such a large force is not controlled, the whole machine will vibrate. It is impossible to achieve perfect imaging. The ASML lithography machine uses the so-called balance mass to absorb the reaction force exerted by the balanced wafer platform on the base, and it is perfectly balanced. The entire machine is completely static and stable.
The wafer has completed extremely precise measurement at the measuring end, and it needs to be perfectly positioned within a very short exposure time. This requires precise mechanical operation and real-time positioning correction. The ASML DUV lithography machine achieves 20,000 measurement corrections per second and confirms precise positioning with sensors that are accurate to 60 picometers (0.06 nanometers, smaller than a silicon atom).
ASML‘s state-of-the-art DUV lithography machine can lithograph more than 6,000 wafers per day, which means more than 600,000 round-trip scans per day. How can it operate 24 hours a day, 365 days a year, and still maintain nanometer accuracy? Won’t the wafer stage wear out? The wafer stage is moved without contact. The ASML wafer stage has two types of levitation technology, the Twinscan XT air levitation method, and the new generation Twinscan NXT magnetic levitation method. By moving without contact, ASML achieves extremely high-speed motion and long-lasting stable operation.
ASML combines precise measurement, precision mechanics, accurate positioning, mastery of light and magnetism, and the perfect use of water to create a unique lithography machine step by step, thus allowing Moore’s Law to continue.
