ASML’s 300mm scanner-systems are built on the TWINSCAN (XT/NXT) platform and yield high productivity levels for dry as well as immersion litho-scanners. NXT:1980Di immersion scanners yield productivity levels as high as 275wph while maintaining the overlay accuracy. The NXT:1980Di can be equipped with a new leveling mode that results in a significant reduction of the time that is spent on measuring the wafer focus height map. In the new leveling mode the focus height map is measured employing the full width of the level sensor and thereby minimizing the number of leveling scans. In this paper we describe the implementation of the LIL-method in the TWINSCAN platform design. Here, we report on the focus / leveling performance for both test as well as customer product wafers, and present a productivity outlook on the performance gain for a selected set of exposure use-cases.
Scanners in High-Volume-Manufacturing conditions will experience a large range of reticles that vary in reticle transmission and reticle diffraction characteristics. Especially under full production loads reticles will heat up due to the exposure light-load and as such experience thermo-mechanical deformations. The resulting reticle pattern distortion can be partially translated in a deteriorated overall system overlay. Due to the geometry of the reticle and exposure fields, these reticle thermal effects are in general barrel-shape distortions that can be well corrected with the available set of lens manipulators. Nevertheless node-over-node the residual overlay errors associated with thermo-mechanical reticle deformation needs further reduction since it contributes to the total onproduct overlay performance. To reduce overlay caused by reticle temperature drift, NXT1980Di includes an active cooling mechanism suppressing the reticle temperature changes during exposure significantly. Even though the reticle temperature excursions are well suppressed, residual intra-wafer overlay drift effect can still be observed. Before exposure of a wafer, reticle deformation is measured during reticle align using in-line alignment / image sensors (TIS or PARIS). This is enabled by adding alignment markers around the circumference of the image field on the reticle. The measured reticle deformations are then fed to the system control network and dynamically corrected for by making use of the available manipulators in the scanner and the projection lens. Wafer-by-wafer reticle distortion measurements are performed to accurately capture the transient dynamics present in reticle heating during normal production lots. A new version of Reticle Heating Feed-forward Control (RHC2) is introduced that uses reticle-heating-induced deformation measurements over time and exposure sequence information to calibrate reticle-deformation-predictionmodels. These models are based on thermo-mechanical models that simulate reticle deformation under various exposure conditions and are applied in-line to the exposures to reduce intra-wafer overlay drift effects.
Adjustment and control of the illumination pupil asymmetry is relevant for wafer alignment and overlay of lithography tools. Pupil asymmetries can cause a tilt in aerial image (Aerial Image Tilt, or AIT). This AIT, combined with a focus offset, leads to a horizontal image shift. Pupil asymmetries can be related to a shift of the entire illumination pupil (geometrical telecentricity) caused by illuminator misalign. Another type of pupil asymmetry is energetic imbalance (quantified by pupil Center of Gravity, COG). The scanner can show pupil variation across the exposure slit.
In general the COG at the edge of the slit is often worse than in the center part of the slit. Recently, ASML has released the NXT:1980Di that is equipped with an enhanced illuminator to improve pupil COG variation across the slit. In this paper we explore the performance of this scanner system and show that the AIT variation across the slit is also reduced significantly.
Overlay is one of the key factors which enables optical lithography extension to 1X node DRAM manufacturing. It is natural that accurate wafer alignment is a prerequisite for good device overlay. However, alignment failures or misalignments are commonly observed in a fab. There are many factors which could induce alignment problems. Low alignment signal contrast is one of the main issues. Alignment signal contrast can be degraded by opaque stack materials or by alignment mark degradation due to processes like CMP. This issue can be compounded by mark sub-segmentation from design rules in combination with double or quadruple spacer process. Alignment signal contrast can be improved by applying new material or process optimization, which sometimes lead to the addition of another process-step with higher costs. If we can amplify the signal components containing the position information and reduce other unwanted signal and background contributions then we can improve alignment performance without process change. In this paper we use ASML's new alignment sensor (as was introduced and released on the NXT:1980Di) and sample wafers with special stacks which can induce poor alignment signal to demonstrate alignment and overlay improvement.
Immersion scanners remain the critical lithography workhorses in semiconductor device manufacturing. When progressing towards the 7nm device node for logic and D18 device node for DRAM production, pattern-placement and layer-to-layer overlay requirements keep progressively scaling down and consequently require system improvements in immersion scanners. The on-product-overlay requirements are approaching levels of only a few nanometers, imposing stringent requirements on the scanner tool design in terms of reproducibility, accuracy and stability.
In this paper we report on the performance of the NXT:1980Di immersion scanner. The NXT:1980Di builds upon the NXT:1970Ci, that is widely used for 16nm, 14nm and 10nm high-volume manufacturing. We will discuss the NXT:1980Di system- and sub-system/module enhancements that drive the scanner overlay, focus and productivity performance. Overlay, imaging, focus, productivity and defectivity data will be presented for multiple tools.
To further reduce the on-product overlay system performance, alignment sensor contrast improvements as well as active reticle temperature conditioning are implemented on the NXT:1980Di. Reticle temperature conditioning will reduce reticle heating overlay and the higher contrast alignment sensor will improve alignment robustness for processed alignment targets.
Due to an increased usage of multiple patterning techniques, an increased number of immersion exposures is required. NXT:1980Di scanner design modifications raised productivity levels from 250wph to 275wph. This productivity enhancement provides lower cost of ownership (CoO) for customers using immersion technology.
Progressing towards the 10nm and 7nm imaging node, pattern-placement and layer-to-layer overlay requirements keep on scaling down and drives system improvements in immersion (ArFi) and dry (ArF/KrF) scanners. A series of module enhancements in the NXT platform have been introduced; among others, the scanner is equipped with exposure stages with better dynamics and thermal control. Grid accuracy improvements with respect to calibration, setup, stability, and layout dependency tighten MMO performance and enable mix and match scanner operation. The same platform improvements also benefit focus control. Improvements in detectability and reproducibility of low contrast alignment marks enhance the alignment solution window for 10nm logic processes and beyond. The system’s architecture allows dynamic use of high-order scanner optimization based on advanced actuators of projection lens and scanning stages. This enables a holistic optimization approach for the scanner, the mask, and the patterning process. Productivity scanner design modifications esp. stage speeds and optimization in metrology schemes provide lower layer costs for customers using immersion lithography as well as conventional dry technology. Imaging, overlay, focus, and productivity data is presented, that demonstrates 10nm and 7nm node litho-capability for both (immersion & dry) platforms.
Mainstream high-end lithography is currently focusing on 32 nm node and 22 nm node where 1.35 NA immersion
technology is well established for the most critical layers. Double-patterning and spacer-patterning techniques have been
developed and are being widely used to print the most critical layers.
Further down the lithography roadmap we see 1x nm nodes coming where EUV lithography will take over critical
layers from immersion. In order to enable a smooth industry-wide transition towards EUV, 1.35 NA immersion
technology will continue to play a critical role in manufacturing front end layers in the coming years. Using immersion
technology beyond the 22 nm node, we expect an increase in the use of double and even quadruple patterning
technology for the critical layers. This demands tighter control of especially overlay and focus performance on the 1.35
NA immersion tools. Also fully flexible illumination and wave front control will be needed to optimize the contrast for
these low k1 applications.
In this paper we present the state-of-the-art system performance of today's 1.35 NA ArF immersion tool production
workhorse, the TWINSCAN NXT:1950i. Furthermore we show the required scanner improvements on imaging, overlay
and cost of ownership to enable device shrink below the 20 nm node in 2013 using immersion technology.