Semiconductor complexity continues to increase with each new process node as the industry pushes the limits of 2D logic and DRAM scaling using EUV lithography and transitions to intricate 3D architectures including Gate-All-Around (GAA) transistors, high-aspect ratio DRAM and 3D NAND memories. This complexity creates significant challenges for process engineers. Defects become much more critical and difficult to detect as chip patterns shrink, the use of 3D structures proliferate and the number of layers increases. The ability to detect and characterize tiny, buried defects in emerging logic and memory chip designs is surpassing the imaging capabilities of conventional eBeam technology. In short, you can’t fix what you can’t see.
This is where cold field emission (CFE) technology comes in. Long regarded as the optimal eBeam technology, CFE operates at room temperature, resulting in narrower, higher-energy electron beams that produce higher resolution and faster imaging speed compared to conventional thermal field emission (TFE) technology which operates at temperatures exceeding 1,500 degrees Celsius. Process engineers can tune the CFE eBeam system for maximum resolution or they can lower the resolution to match that of TFE but with significantly faster imaging speed. The pictures below illustrates the resolution and speed differences between CFE and TFE.
Until now, use of CFE-based systems has been limited to lab environments because the stability of the eBeam column was insufficient for the stringent requirements of high-volume semiconductor manufacturing. One of the main factors affecting stability is the cleanliness of the eBeam column. All eBeam systems contain an extremely small source tip that emits the electrons used for imaging. Because the tip is so small, any contamination – even a single atom – can potentially disrupt the flow of electrons and cause system instability.
In TFE systems, the source tip is so hot that it automatically repels any contaminants that might gather on the surface. In contrast, the room temperature operation of CFE, which is the main driver for its higher performance, makes cleaning much more challenging.
To bring CFE technology out of the lab and into the fab, Two innovations that solved the CFE stability challenge. The first was creating an extreme ultra-high vacuum inside the eBeam column that is well below 1 x 10-11 millibar, which is two to three orders of magnitude better than for TFE systems and nearly the vacuum level found in outer space! Through an extensive optimization process, an extreme ultra-high vacuum was combined with specially developed chamber materials to greatly reduce the presence of contaminants inside the eBeam column.
Even under extreme ultra-high vacuum, a tiny amount of residual gas can still exist. If gas molecules adhere to the electron source, performance is significantly degraded. The second innovation we developed is a cyclical self-cleaning process that continuously removes contaminants from the CFE source, thereby enabling stable and repeatable performance for high-volume manufacturing environments.
Bringing CFE eBeam technology to high volume chip making manufacturing took more than 10 years of intense development of a CFE defect review and defect inspection platforms.
By enabling chipmakers to discover defects they’ve never been able to see before, faster, CFE is helping accelerate the development and production of advanced process nodes.
Extreme Ultra Violet Lithography (EUVL) is a major patterning solution candidate being
considered for the ITRS (International Technology Roadmap for Semiconductors)
advanced technology nodes commencing with the 22nm Half Pitch (HP) nodes.
Achieving defect free EUVL masks is a critical issue in the wafer manufacturing process
and thus the importance for mask inspection technology to be ready to support pilot line
development.
EUV mask inspection presents additional challenges with smaller line width, multilayer
defects and no pellicle to protect the mask. In addition, Line Edge Roughness on the
mask can limit the detection sensitivity. Configurable inspection illumination conditions
were considered to enhance the contrast of the mask image and improve the detection
sensitivity.
Here we present experimental results of evaluating the defects detecting capability on
several EUVL masks of different technology nodes. EUVL mask inspections were done
using Material's Aera3TM DUV (193nm) reflected illumination optical inspection system
employing configurable inspection illumination conditions and magnifications.
Scanner introduction into the fab production environment is a challenging task. An efficient evaluation of scanner
performance matrices during factory acceptance test (FAT) and later on during site acceptance test (SAT) is crucial for
minimizing the cycle time for pre and post production-start activities. If done effectively, the matrices of base line
performance established during the SAT are used as a reference for scanner performance and fleet matching monitoring
and maintenance in the fab environment.
Key elements which can influence the cycle time of the SAT, FAT and maintenance cycles are the imaging, process and
mask characterizations involved with those cycles.
Discrete mask measurement techniques are currently in use to create across-mask CDU maps. By subtracting these maps
from their final wafer measurement CDU map counterparts, it is possible to assess the real scanner induced printed errors
within certain limitations. The current discrete measurement methods are time consuming and some techniques also
overlook mask based effects other than line width variations, such as transmission and phase variations, all of which
influence the final printed CD variability.
Applied Materials Aera2TM mask inspection tool with IntenCDTM technology can scan the mask at high speed, offer full mask coverage and accurate assessment of all masks induced source of errors simultaneously, making it beneficial for
scanner qualifications and performance monitoring.
In this paper we report on a study that was done to improve a scanner introduction and qualification process using the
IntenCD application to map the mask induced CD non uniformity. We will present the results of six scanners in production and discuss the benefits of the new method.
Scanner performance is influenced by the quality of its illumination, mechanical and optical elements and the impact of
these factors on the printed wafer. Isolation of the aggregated scanner errors from other sources of error on the printed wafer
is a challenging task since the total error budget of the lithography process consists of many dynamic sources, such as wafer
planarity and film stack properties.
The mask is conceptually part of the scanner optics and integral to the imaging process. Therefore the mask error
contribution to the overall error becomes relevant for any advanced lithography process.
Discrete mask measurement techniques are currently used to create across mask CDU maps. By subtracting these maps from
their final wafer measurement CDU map counterparts, it is possible to assess within certain limitations the real scanner
induced printed errors. The current discrete measurement methods are time consuming and some overlook errors other than
linewidth variations, such as transmission and phase variations, all of which influence the final printed CD variability.
In this paper we present a methodology, which leverages Applied Materials Aera2tmmask inspection tool, based on a socalled
IntenCDtm technology.
IntenCD aerial imaging produces maps by scanning the mask at high speed, offer full mask coverage and accurate
assessment of all mask induced errors simultaneously, making it ideal for mask CDU characterization and scanner
qualification.
With each successive technology node the overlay specifications of the immersion lithography scanner have become
increasingly more stringent. One of the challenges is high order distortions introduced by the mask. These distortions may
contribute significantly to the product overlay budget raising it above the specification requirements and are not easy to
correct.
The higher order distortions, originating from pellicle and mask process imperfections, have been shown to result in errors
in the range of several nanometers to the overall overlay budget [1],[2]. Correction markers and the actual product features
cannot occupy the same space on the mask. As a result they might be exposed to differing local distortions which could
result in non-optimal systematic distortion corrections [3]. Therefore high precision placement measurements of features
across the mask are required for placement control and correction.
The Applied Materials Aera2TM aerial imaging mask inspection system is capable of generating high precision global and
local feature placement maps with a high measurement density. These maps can be used to monitor feature placement.
Furthermore, the maps can be used in a feed forward APC system such as ASML's GridMapper IntrafieldTM[4]. This feed
forward system helps to reduce the overall overlay error of feature processes and to meet the stringent overlay budget
requirements.
In this paper we present for the first time (?) mask registration results obtained with the Aera2 and show that this tool is able
to meet the 1 [nm], 3δ ITRS requirement [5] for the 22nm node.
Key words: Inspection, Mask, Reticle, Placement, Registration, Overlay,
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