Backside defects a few micrometers in size are serious concern in lithography because they can degrade the image
quality on a wafer. It was known that defects attached on the backside affected the printing images on a wafer by locally
altering the partial coherence (σ) and the transmitted intensity of the illumination. The ability to detect and to simulate
their impact of defects on the backside is one of the key components in ensuring quality of photomask.
The purpose of this study is to determine the minimum size of defects on the backside which would be affected
printability in 193nm photolithography. It was investigated to the influence of wafer critical dimension (CD) variation
according to illumination and NA, that of refraction according to defect size.
For this study, a reticle was designed to include line and space patterns, contact patterns and isolated patterns on the front
side. And the type of defects attached on the backside was made of chrome to investigate the relation between
transmittance of backside defects and its printability.
The correlation of measurements made with UV and DUV-based inspection system; simulation performed with a
193nm aerial image measurement system. Besides the allowable size of backside defects was determined using the
criterion of a maximum intensity variation of 10%.
We have investigated the factors having influence on the lithographic fidelity variation in 193nm masks. Significant
researches have been studied that haze contamination, resulting from the absorption of chemical residual ions and mask
container out-gassing in mask fabrication, is one of the major component to reduce the optimized lithography condition
such as Best Focus, Depth of Focus and Exposure latitude of individual feature. And also environment being containing
humidity, ambient AMC (airborne molecular contamination) react with high exposure energy to form crystal growth of
ionic molecular complex such as ammonium sulfate causing abnormal printability. Moreover, optical issue of organic
pellicle membrane is thoroughly considered that perfluoro polymer degradation induced by high photon energy affect the
transmittance intensity. Consequently, these photophysical alterations bring about the lithographic variation and cause
considerable defects in wafer printing.
In this paper, we tried to verify the influence grade inducing the lithographic variation among the latent contamination
factors consisting of mask back-side quartz contamination, the growth of exposure energy based haze phenomena, thin
organic pellicle membrane degradation and modified character of MoSiN surface. Metrological inspection and
photochemical reaction evaluations were conducted with several equipments including AIMS, Scatterometer, XPS, SIMS,
FT-IR, UV, ArF acceleration laser to demonstrate the proposal mechanism of correlation between lithographic variation
and latent contamination factors. The optical issues and lifetime of ArF PSM were simulated with the evaluation of
effects of pellicle degradation and surface modification.
In sub-60nm technology node, cleaning process becomes specialized to clear the defects without pattern damage as
decreasing critical particle size to control. While cleaning process has to meet the primary requisite, removal of particle
including organic residue and prevention of particle re-deposition, it should enable to suppress haze phenomena for a
long life of photomask. However, to solve the problem of haze, the chemical materials caused haze seed should be
hardly used and physical force becomes strengthen as the compensation for cleaning efficiency. Unfortunately it brings
about another problem, pattern damage seriously.
In this paper, adequate cleaning conditions which are applicable in sub-60nm technology node are evaluated to meet the
dilemma among three requirements, high cleaning efficiency, and prevention of pattern collapse, and prevention of haze
phenomenon. All cleaning steps in photomask process were set up using only 172nm UV irradiation for degradation of
organic contaminants and deionized water (DI) with acoustic power for particle lift-off. The effect of UV and DI
cleaning on cleaning efficiency and haze phenomena was derived from carrying out chemical and physical analysis
simultaneously. Also, we could quantify the statistical probability of pattern collapse in each of technology node and
layer shape as different condition of megasonic frequency and its power. As a result, it was known that this cleaning
process have various merits to make out dilemma mentioned above, if it satisfies optimized conditions.
As the specification for photomask becomes tighter, it is strongly demanded for achieving precise CD MTT (critical
dimension mean to target) and enhanced defect controllability in photomask fabrication. First of all, it is necessary that
reducing the factors of CD MTT error and introducing the reliable method to correct CD error for accurate CD
requirement of attenuated PSM (phase shift mask). From this point of view, one of CD correction methods which consist
of Cr CD measurement step after resist strip (strip inspection CD: SI CD) and additional corrective Cr dry etch step was
developed. Previous SI CD correction process resulted in accurate CD control within the range of CD MTT. However it
was not appropriate for defect control due to additional resist processes for selective protection of Cr pattern during CD
correction process.
In this study, the method for achieving precise CD MTT by correcting CD error without any resist process is
investigated. It is not suitable for the CD correction process to control CD MTT precisely that Cr etched resist (etch
inspection CD: EI CD) is very vulnerable to E-beam scanning during CD measurement. Otherwise, photoresist after Cr
etch selectively shrinks via UV irradiation under ozone (O3) condition, which drives a reduction of CD MTT error as a
result of accurate CD measurement (UV-irradiation inspection CD: UI CD). Moreover, it is not necessary any resist
process for Cr protection due to UV irradiated resist as enough for a etch barrier. It is a strong advantage of novel CD
correction method. This strategy solves the problems such as both CD measurement error on the EI CD correction
method and defects originated from resist process on the SI CD correction method at once. For the successful
incorporation of UI CD correction method, several items related with CD should be evaluated: accuracy and repeatability
of CD measurement under UI CD, control of CD MTT and CD uniformity, additional corrective etch bias for UI CD,
independence of corrective Cr etch process from UV irradiated resist, isolated-dense CD difference,.. etc. In this paper,
strategy of design for the progressive CD correction method for defect-free photomask and process details will be
discussed.
The ability to eliminate the critical source of haze contamination which can be derived from the cleaning chemistry residues and mass production environment has become a major challenge for 193 nm photolithography in semiconductor industry. Furthermore, as the specification for pattern generation on photomask becomes tighter, it is getting harder and harder to eliminate defects with both minimal structural damage and preservation of photophysical properties. We designed for the smart cleaning strategy to achieve the defect-free photomasks as a concern of above current issue with a combination of well-known cleaning technology, such as using the collective effects of ozonated water (DIO3) for the alternative to conventional clean (SPM/SC1) and UV/O3 treatment for the control of sulfate concentration. In addition to photomask clean, these strategies are also used for photoresist stripping.
As well as the final cleaning process, it is a rational strategy that judicious modification of inter-process clean. Specially, that kind of view is focused on the after-development clean (ADC) process which mainly eliminated the source of fatal defects on the mask, such as pattern bridge following dry etch process.
In this paper we will propose a novel cleaning strategy for the elimination of potential source of haze formation and fatal defects.
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