As the semiconductor industry transitions to 10 nm node production, there is an emphasis on ensuring optimal equipment performance and productivity under the complex process conditions typical of the high volume manufacturing environment (HVM). At the recent LithoVision and SPIE Advanced Lithography conferences, Nikon experts spotlighted comprehensive solutions that ensure optimal overlay and focus performance under the extreme process conditions experienced in HVM.
Yuichi Shibazaki, Nikon Corporation Technical Director, reported at LithoVision that under test exposure conditions at 250 wafers per hour, leading-edge immersion scanners such as the Nikon NSR-S630D provide single machine overlay (SMO) below 1.7 nm, mix-and-match overlay (MMO) less than 2.5 nm, and across wafer focus uniformity 3σ below 10 nm (Figure 1A). While the result of those tests is quite good, it’s also important to chipmakers to maintain a comparable level of performance and productivity with product wafers under challenging HVM processing conditions.
|Figure 1A. Shibazaki showed excellent S630D performance data at 250 wafers per hour (left image). Figure 1B. Shibazaki explained that lens and reticle heating are the dominant detractors for on-product performance.|
Shibazaki explained that lens and reticle heating are the dominant detractors for on-product performance (Figure 1B). He showed that, left uncorrected, the shot shape can deviate as much as 19 nm and uncorrected lens aberrations can induce a noticeable focus shift as a result of lens and reticle heating effects. Additionally, odd order thermal aberrations can induce overlay metrology errors due to differences in device pattern and overlay mark placement in the resist, making precise wavefront control crucial to on-product overlay (OPO) as well.
Leading-edge multiple-patterning applications employ a wide range of illumination conditions, exposure doses, and reticle transparency levels (Figure 2A). Scanner lens heating is proportional to the resist dose D・η, and reticle heating is proportional to D・(1-η). Noting that reticle transmission (η) can range from 5% to 90%, and that illumination patterns can be anything from a large conventional pattern to a thin dipole, Shibazaki pointed out that lot-to-lot control of the lens now poses a challenge. To maintain high levels of performance, accurate feed forward control is imperative.
To illustrate that this challenge is being met, Shibazaki reported on a series of tests that show the NSR-S630D immersion scanner controls lens heating precisely and corrects reticle expansion effectively (Figure 2B). He noted that excellent OPO (below 2.5 nm Avg.+ 3σ) was maintained during extreme “combination” overlay where the first exposure used annular illumination and η=5%, followed by a second exposure using dipole X illumination and η=50%. In addition, tool performance in a preheated hot start condition was also verified and across lot OPO was below 2.7 nm.
|Figure 3A. Shibazaki discussed the benefits of computational layer setup (left image). Figure 3B. He described an example where the Z5 factor for dipole illumination was effectively suppressed using computational setup with on-product learning.|
Shibazaki discussed the benefits of computational layer setup, which takes into consideration factors such as the mask pattern, pupil fill, and lens wavefront (Figure 3A). He commented that on-product learning can also be employed to further improve performance in manufacturing, and noted that this can be achieved with minimal impact on scanner productivity. He described an example where the Z5 factor for dipole illumination was effectively suppressed using computational setup for predictive correction, coupled with high speed multipoint phase measurement interferometry (PMI) for on-product learning adjustment (Figure 3B). Verification data collected using conventional illumination (η=5%), for the first exposure and dipole Y illumination (with 90% reticle transparency) for the second exposure achieved OPO < 2.5 nm across the lot.
At SPIE Advanced Lithography later that week, Hirotaka Kono, Chief Researcher for Nikon Corporation, provided a more in-depth view of the ways Nikon is optimizing tool performance under production conditions. He explained that an integrated ecosystem approach is needed to support HVM at 10 nm and beyond. He announced the Nikon Open Platform Solution is built around the Plug and Play Manager, and involves partnership with a number of EDA, track, and metrology suppliers to deliver a comprehensive lithography solution to customers. The Plug and Play Manager supports a number of applications that help ramp production as quickly as possible (Figure 4). These Smart Start functions, which include CDU, Overlay, and Lens Masters, provide fast and accurate layer recipe setup to optimize tool performance on product.
|Smart Start Application||Purpose||Example Benefits|
|CDU Master||CD uniformity improvement |
Dose & focus correction
|35~40% improvement in CDU on customer product|
|Lens Master||Thermal aberration optimizer|
|Z5 coefficient over time reduced from max of 89ml to <10ml|
|Overlay Master||OVL stability improvement |
|S630D MMO below 2.3 nm|
|Smart Running Application||Purpose||Example Benefits|
|Zeroing XY||Enhanced grid stabilization|
|3 week SMO stability improved from ~2 nm to below 1.8 nm (6 lots)|
|Zeroing AF||Optimized focus stability|
|Customer 3σ data reduced by ~30% across 1 month period|
|Figure 4. The Plug and Play Manager supports a variety of Smart Start and Smart Running applications to deliver a comprehensive litho solution to customers.|
Kono also described the Smart Running functions, Zeroing XY and Zeroing AF. These automated periodic machine calibration functions enhance scanner stability with machine-specific grid (XY) and autofocus (AF) corrections. He shared sample data that showed Zeroing XY improved S630D long-term overlay performance from ~2 nm (Avg.+3σ) to below 1.8 nm across a 3 week period (Figure 5A). Customer data shown at LithoVision (Figure 5B) validated the stability benefits of Zeroing AF as well.
|Figure 5A. Zeroing XY improved S630D long-term overlay performance from ~2 nm (Avg.+3σ) to below 1.8 nm across a 3 week period (left image). Figure 5B. Customer data shown at LithoVision validated the stability benefits of Zeroing AF as well.|
Leading-edge photolithography requires comprehensive manufacturing solutions, as well as excellent hardware performance. The Nikon NSR-S630D immersion scanner delivers single machine on-product overlay less than 3 nm across a variety of extreme manufacturing conditions. When combined with computational layer setup and on-product learning, this enables chipmakers to satisfy even their most demanding production requirements.