Extreme Ultraviolet Lithography (EUVL) is one of the leading candidates for the next-generation lithography in the sub-30 nm regime. Stringent flatness requirements have been imposed for the front and back surfaces of EUVL masks to ensure successful pattern transfer that satisfies the image placement error budget. The EUVL Mask Standard (SEMI P-37) specifies the flatness of the two mask surfaces to be approximately 50 nm peak-to-valley. It is essential to measure the mask surface nonflatness accurately (without gravitational distortions) to the extent possible. The purpose of this research was to study the various mask mounting techniques and to compare these methods for repeatability and accuracy during the measurements.
The use of an electrostatic chuck to support and flatten an extreme ultraviolet (EUV) mask during scanning exposure
will be a critical component to meet the stringent requirements on image placement errors in the sub-30-nm regime.
Consequently, the ability to predict the response of the mask during e-chucking is necessary for the design and
implementation of the e-chuck system. This research focuses on characterizing the coefficient of friction between the
EUV reticle and the dielectric material of the chuck. A customized tool was constructed to test chuck and reticle
samples both in air and in a vacuum chamber. Studies were conducted to identify the friction coefficient at various
chucking pressures and to examine the effects of wear caused by repeated measurements on the same location of the
reticle surface. All experiments were performed in a cleanroom environment. Results of the friction testing illustrate the
range of values to expect for typical EUV reticles and chucks. Finite element (FE) modeling was then used to illustrate
the effects of friction on the overall capability of the chuck to flatten the mask. Additional FE simulations demonstrated
the magnitude of the friction force needed to ensure that the reticle would not slip during the acceleration / deceleration
loading seen in the scanning exposure process.
KEYWORDS: Extreme ultraviolet lithography, Interferometry, Photomasks, Reticles, Data modeling, 3D modeling, Mathematical modeling, Photography, Image quality, Control systems
Extreme Ultraviolet Lithography (EUVL) is one of the leading candidates for Next-Generation Lithography in the sub-45-nm regime. Successful implementation of this technology will depend upon advancements in many areas,
including the quality of the mask system to control image placement errors. For EUVL, the nonflatness of both the
mask and chuck is critical, due to the nontelecentric illumination during exposure. The industry is proposing to use an
electrostatic chuck to support and flatten the mask in the exposure tool. The focus of this research is to investigate the
clamping ability of a pin-type chuck, both experimentally and with the use of numerical simulation tools, i.e., finite
element modeling. A status report on electrostatic chucking is presented, including the results obtained during
repeatability studies and long-term chucking experiments.
According to the International Technology Roadmap for Semiconductors, meeting the strict requirements on image
placement errors in the sub-45-nm regime may be one of the most difficult challenges for the industry. For Extreme
Ultraviolet Lithography (EUVL), the nonflatness of both the mask and chuck is critical as well, due to the
nontelecentric illumination during exposure. To address this issue, SEMI Standards P37 and P40 have established the
specifications on flatness for the EUVL mask substrate and electrostatic chuck. This study investigates the procedures
for implementing the Standards when measuring and characterizing the shapes of these surfaces. Finite element
simulations are used to demonstrate the difficulties in supporting the mask substrate, while ensuring that the measured
flatness is accurate. Additional modeling is performed to illustrate the most appropriate methods of characterizing the
nonflatness of the electrostatic chuck. The results presented will aid in identifying modifications and clarifications that
are needed in the Standards to facilitate the timely development of EUV lithography.
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