This paper presents an extended Abbe based imaging algorithm for faster and more accurate simulations of current and future projection lithography systems The basics of the physical model and several methods for the evaluation of the new image simulation software are explained The comprehensive evaluation of the new image simulation software includes convergence tests, comparisons with analytical results, and various methods for the assessment of computed imaging results in terms of intensity difference plots, simulated linewidths, and image slopes Tests include simulations for two- and three-dimensional thin and rigorous simulated masks, scalar and vectorial computations of intensity distributions in air/immersion liquid (aerial images) and photoresist (bulk images), respectively The test scenarios range from special settings which result in simple two-beam interferences to large area simulations of more complex mask layouts The excellent accuracy and computational performance of the new imaging algorithm is demonstrated by a comparison with the well-established imaging algorithm of Fraunhofer IISB The new imaging algorithms are integrated in the research and development lithography simulator DrLiTHO of Fraunhofer IISB

Extended Abbe approach for fast and accurate lithography imaging simulations

EUVL is the strongest candidate for a sub-20nm lithography solution after immersion double-patterning. There are still
critical challenges for EUVL to address to become a mature technology like today's litho workhorse, ArF immersion.
Source power and stability, resist resolution and LWR (Line Width Roughness), mask defect control and infrastructure
are listed as top issues. Source power has shown reasonably good progress during the last two years. Resist resolution
was proven to resolve 32nm HP (Half Pitch) lines and spaces with good process windows even though there are still
concerns with LWR. However, the defectivity level of blank masks is still three orders of magnitude higher than the
requirement as of today.
In this paper, mask defect control using wafer inspection is studied as an alternative solution to mask inspection for
detection of phase defects on the mask. A previous study suggested that EUVL requires better defect inspection
sensitivity than optical lithography because EUVL will print smaller defects. Improving the defect detection capability
involves not only inspection system but also wafer preparation. A few parameters on the wafer, including LWR and
wafer stack material and thickness are investigated, with a goal of enhancing the defect capture rate for after
development inspection (ADI) and after cleaning inspection (ACI). In addition to defect sensitivity an overall defect
control methodology will be suggested, involving mask, mask inspection, wafer print and wafer inspection.

The analysis of EUV mask defects using a wafer defect inspection system

Assessing the printability of EUV (extreme ultraviolet) lithography mask pattern defects is critical for determining EUV
mask patterning, defect metrology, and repair technology requirements. Printability of mask defects at the wafer level
depends on defect size, defect shape, defect location, and the line width and pitch of the structure being printed. Earlier
reports showed the relationship between the defect size on the mask and the printed critical dimension for 40-70 nm
dense lines. Improvements in the EUVL process now enable assessment of mask pattern defect printability for 22-40
nm half-pitch features.
We report here the smallest mask pattern defects that printed at different locations in 22-40 nm structures using the Intel
Micro-Exposure Tool (MET). Various types of defects such as indentations or protrusions were purposely incorporated
into features on an EUV mask. The sizes of the patterned defects on the mask were drawn between 10-250 nm (= 2-50
nm on the wafer). The minimum printable defect size varied by over 100 nm, depending on the defect shape and
location.

Printability of extreme ultraviolet lithography mask pattern defects for 22-40 nm half-pitch features

Availability of defect-free masks is one of the most critical issues for enabling EUV lithography. Among others,
multilayer phase defects embedded in EUV blanks are primary concern because multilayer defects as low as 2nm are
expected to be printable. Therefore, thorough understanding of defect printability through experiments is anticipated
to derive critical sizes of multilayer defects. A test mask with programmed multilayer defects has been fabricated for
this purpose with various sizes of defects as well as various protrusion sizes of defects from the absorber lines. As the
first step toward rigorous printability prediction that can comprehend any arbitrary-shaped multilayer defects, line
defects parallel to absorber lines have been selected for printability experiments using micro exposure tool (MET) in
Lawrence Berkeley National Lab. for direct comparison with 2D simulation. The simulation of electro-magnetic field
with multilayer defects has been carried out using finite-element-method with triangular cells that well match the needs
for incorporation of minute change in multilayer profile as observed in a TEM photograph. The experiments have
proved that there is systematic correlation between the sizes of defect protrusion from beneath the absorber line with
printed line CD. The defect with protrusion size of 33nm on mask, however, did not show any evidence of defects
while aerial image simulation indicated there should be detectable difference in print results. Root cause investigation
for this gap indicated that the multilayer bump affected the absorber width and profile above and hence generated the
gap between experiments and simulation that assumed identical absorbers. Simulation with exact absorber shape input
well matched experimental results.

EUV exposure experiment using programmed multilayer defects for refining printability simulation

Mask defects are of increasing concern for future lithography generations. The improved resolution capabilities of immersion and EUV systems increase also the sensitivity of these systems with respect to small imperfections of the mask. Advanced mask technologies such as alternating phase shift masks (AltPSM), chromeless phase shift lithography (CPL), or "thick" absorbers on EUV masks introduce new defect types. The paper presents an application of rigorous electromagnetic field modeling for the study of typical defect printing mechanisms in ArF immersion lithography and in EUV lithography. For standard imaging and mask technologies, such as binary masks or attenuated phase shift masks, small defects usually print as linewidth or critical dimension (CD) errors with the largest effect at best focus. For AltPSM, CPL masks, and EUV masks this is not always the case. Several unusual printing scenarios were observed: placement errors due to defects can become more critical than CD-errors, defects may print more critical at defocus positions different from the center of the process window, the defect printing may become asymmetric through focus, and the risk of defect printing depends on the polarization of the used light source. Several simulation examples will demonstrate these effects. Rigorous EMF simulations in combination with vector imaging simulations are very useful to understand the origins of the observed defect printing mechanisms.

Mask defect printing mechanisms for future lithography generations

Defect-free masks are one of the top issues for enabling EUV lithography at the 32-nm node. Since a defect-free process cannot be expected, an understanding of the defect printability is required in order to derive critical defect sizes for the mask inspection and repair. Simulations of the aerial image are compared to the experimental printing in resist on the wafer. Strong differences between the simulations and the actual printing are observed. In particular the minimum printable defect size is much larger than expected which is explained in terms of resist resolution. The defect printability in the current configuration is limited by the resist process rather than the projection optics.

Thomas Graf

Papers

Defect printability study using EUV lithography

Mask defect printing mechanisms for future lithography generations