Defects within the multilayer mirrors of EUV photomasks have been a leading challenge for EUV lithography for quite some time. By creating non-planar surfaces, they distort both the amplitude and phase of reflected light. Amplitude errors generally create a CD error on wafer, whereas phase errors tend to cause asymmetric printing through focus. Since defect-free mask blanks are not expected to be available for initial high volume EUV manufacturing, defect mitigation, compensation, and repair strategies are essential. This paper describes a technique to repair both the amplitude and phase effects of multilayer defects. For a bump defect, the phase effect (i.e. tilted Bossung curve behavior) is corrected by removing multilayer material in the vicinity of the defect. This creates a phase effect opposite to that of the defect and the two effects cancel. The amplitude error (i.e. CD error) caused by both the defect and by the phase repair is then corrected by modifying the surrounding absorber pattern. The repairs in this paper are performed by nanomachining with an AFM repair tool. The concept is validated by a combination of simulation and experimental studies with data from the Actinic Inspection Tool (AIT) at the Lawrence Berkeley National Laboratories, the EUV Alpha Demo Tool (ADT) in Albany, New York, and an AFM repair tool. The process for a complete multilayer repair is described using an example native defect repair. Encouraging results indicate that nanomachining is capable of creating the complex nano-scale three dimensional topographies required for the repair. Repair strategies for both bump and pit defects are addressed. Multiple simulation studies are used to understand the requirements for such a repair and what type of repairs may be possible.
The technology to selectively remove nanoparticles from a photomask surface by adhering it to an AFM tip
(BitClean) first introduced with the Merlin® nanomachining mask repair platform has been successfully integrated
in numerous mask house production centers across the globe over the last two years. One outstanding request for
development from customers has been to develop the capability to not only selectively remove nanoparticles from a
target surface, but to also redeposit in another target region. This paper reviews the preliminary work done to
develop this capability with particular emphasis on its potential applications in creating realistic nanoparticle
inspection sites for KLA systems at critical pattern print locations as well as the accumulation of trace amounts of
contaminates for better compositional and print-impact analysis. There is also a feasibility study of new ultra-high
aspect ratio (AR > 1.5) NanoBits for future BitClean process applications. The potential for these capabilities to be
adapted for new applications will be examined for future work as well as a detailed parametric process analysis
with the goal of showing how to make significant improvements in BitClean PRE.
The haze nucleation and growth phenomenon on critical photomask surfaces has periodically gained
attention as it has significantly impacted wafer printability for different technology nodes over the
years. A number of process solutions have been shown to suppress or minimize the propensity for
haze formation, but none of these technologies has stopped every instance of haze. Additionally, the
management of photo-induced defects during lithography exposure is expensive, so some capability
will always be needed to remove haze on photomasks for long term maintenance over a mask's
lifetime.
A novel technology is reviewed here which uses a dry (no chemical effluents) removal system to
safely sweep the entire printable region of a pelliclized photomask to eliminate all removable haze.
This process is safe regardless of the mask substrate materials or the presence of small critical
patterns such as SRAF's that may represent damage problems for traditional cleaning methods.
Operational process techniques for this system and performance in removal will be shown for haze
located on the mask pattern surface. This paper will also discuss the theory of operation for the
system, including expected chemical reactions and address the reformation rate of haze crystals.
Data from tool acceptance and preliminary production use will also be reviewed including analysis
of process window through a focus-exposure matrix, repair durability, CD performance, and sort
yield.
A number of new RET's have come to significant adoption in advanced lithography recently, extending technology
trends that have allowed the use of 193 nm wavelengths for nodes well beyond their intended limits. These
enhancement technologies include Computational Lithography (CL) techniques such as Source-Mask Optimization
(SMO), and use of innovative materials such as Opaque MoSi on Glass (OMOG). These new techniques are of particular
focus for examination of their applicability to nanomachining photomask repair. Historically comparative repair results
are shown for the OMOG absorbers which can contain a multi-layer potentially in combination with quartz over-etching
for phase correction. The implementation of nanomachining for CL/SMO photomasks encompasses a larger set of new
technologies introduced in a nanomachining repair tool. These include tip shape de-convolution for improved accuracy
and reproducibility of large complex patterns - many of which are non-orthogonal, and automated import of mask design
data to seed the repair polygon for a pattern which may be unique on the entire mask area (i.e. no pattern reference).
A number of new technologies and processes have been developed for deep ultraviolet (DUV) wavelength
and femtosecond pulsed laser repair of photomasks. These advances have been shown to improve and extend the
repair of both pelliclized and non-pellicilized photomasks for both hard and soft (or nano-particle) in exhaustive
testing at the factory and the end-user site. However, even the best testing is only a simulation of what a repair
tool will see when brought into full production. The purpose of this work is to review some of the knowledge and
experience gained in bringing the repair processes defined with manufactured defects to the more variable defects
encountered in the real world. The impact of the repair technology on increases in mask house throughput and
decrease in costs will also be compared to other (another laser and an advanced FIB) repair tools.
Regardless at what technology node it will be implemented, extreme ultraviolet (EUV) lithography appears to be the
most likely candidate to succeed 193 nm wavelength lithography. However, EUV photomasks present new and different
challenges for both repair and clean processes. Among these are different and more complex materials, greater
sensitivity to smaller topography differences, and lack of pelliclization to protect critical pattern areas. Solutions
developed and recently refined to meet these challenges are reviewed as an integrated solution to make the manufacture
and maintenance of this mask type feasible. This proven, integrated solution includes nanomachining, BitClean® and
cryogenic clean processes applied for hard (missing pattern) and soft (nanoparticle) defect removals with no damage to
underlying multilayers.
Makers and users of advanced technology photomasks have seen increased difficulties with the removal of persistent, or
stubborn, nano-particle contamination. Shrinking pattern geometries, and new mask clean technologies to minimize
haze, have both increased the number of problems and loss of mask yield due to these non-removable nano-particles.
A novel technique (BitCleanTM) has been developed using the MerlinTM platform, a scanning probe microscope system
originally designed for nanomachining photomask defect repair. Progress in the technical development of this
approach into a manufacture-able solution is reviewed and its effectiveness is shown in selectively removing adherent
particles without touching surrounding sensitive structures. Results will also be reviewed that were generated in the
qualification and acceptance of this new technology in a photomask production environment. These results will be
discussed in their relation to the minimum particle size allowed on a given design, particle removal efficiency per pass
of the NanoBitTM (PREPP), and the resultant average removal throughput of particles unaffected by any other available
mask clean process.
Contrary to the prior assumptions of its technical demise, deep UV (DUV) femtosecond pulsed laser repair of
photomasks is continuing to mature and improve as a technology. Similar to the optical enhancements that allow
for 193 nm wavelength light to continue being used down to the 32, or even in some cases 22 nm, node, the process
regimes for this type of laser repair continue to expand as new processes are discovered. This work reviews the
qualification of repair performance for production at a major wafer foundry site. In addition advances are shown in
the area of through-pellicle repair (TRP) process development. These advances include the preferential (versus
surrounding reference mask structures) removal of soft defects and the capability to remove or manipulate particles
on top of a flat absorber region with no detectable removal of the absorber. These developments will further
demonstrate the progressive decoupling of the laser repair spot size from the minimum technology node for laser
repair.
Patterns which are not aligned to standard orthogonal (x and y ordinate) directions have recently been developed
for advanced lithography nodes. Efforts have been successful in developing single pass nanomachining repair
processes to meet the printability requirements for these patterns. This development makes use of the latest
improvements made to the COBRA repair process (the Enhanced COBRA process typically completed in less than
2 minutes of repair time) with symmetric NanoBits to repair opposing critical edges of bridging defects. It also
required fundamental changes in the software tools to allow automated detection of the angle of the edges and the
application of pre-programmed repair edge biases normal (90°) to the detected angled edges. Additionally, some
other new improvements (hardware, software, and process) are reviewed in light of more traditional nanomachining
repairs.
A developing reality with advancing DUV femto-second pulsed laser repair of advanced photomasks after
pelliclization is a significant operational cost savings. This economic advantage comes from the elimination of a
previously critical inspection step before mounting the pellicle on the mask. The laser repair processes developed
allow reduce the number of times masks are cycled through mask inspection tools and prevent recontamination of
the mask surface due to handling during and after the inspection before pelliclization. The latter would require,
without effective through-pellicle repair, removal of the pellicle, repair and/or reclean, then re-inspection and
re-pelliclization of the mask (assuming success with the first cycle of repair and/or reclean). Meanwhile, the cost
and lack of feasibility for this additional process both increase the investment in the mask while at the same time
bringing it closer to being completely scrapped. Recently developed processes that allow for effective
through-pellicle laser repair on advanced photomask technologies (sub 65 nm node) are reviewed, that have
economic advantages and also make the repair of lower-end (and higher volume) photomasks significantly more
profitable to the typical mask maker.
A persistent industry problem impacting photomask yield and costs has been haze formation. The haze nucleation
and growth phenomenon on critical photomask surfaces has periodically gained attention as it has significantly
impacted wafer printability for different technology nodes over the years. A number of process solutions have
been promoted in the semiconductor industry which has been shown to suppress or minimize the propensity for
haze formation, but none of these technologies can stop every instance of haze. Thus some capability will always
be needed to remove haze on photomasks with their final pellicles mounted both at the manufacture and long term
maintenance stages of a mask's lifetime. A novel technology is reviewed here which uses a dry (no chemical
effluents) removal system to sweep the entire printable region of a pelliclized photomask to eliminate all removable
haze regardless of the mask substrate materials or the presence of critical patterns. An operational process
technique for this system and performance in removal is shown for haze located on the mask pattern surface.
Finally, preliminary data from tool acceptance and preliminary use in a production environment will also be reviewed.
The AFM-technology based technique of nanomachining has been well-proven in the area photomask repair since
its introduction a decade ago. However, the problems and challenges facing the mask repair operator have
changed significantly in this period, and ongoing engineering platform development has reflected these shifts, as
well as refinements based on specialized experience with nanomachining repair technology. Improvements from
this technical development include improved monitoring and control of the internal tool environment (to minimize
AFM scan noise and thermal drift), and automation to easily and reliably clean and characterize the 3-dimensional
shape of the NanoBitTM apex. For repair applications, improvements will be shown for the automated and
operator-intuitive reconstruction of 3-dimensional nanometer-scale patterns on the photomask with referenced
z-depth and xy alignment regardless of pattern orthogonality. Multiple pattern repair capability is also reviewed
due to a greater diversity of available process options and multi-repair box capability with a common quartz-level
z-reference point. Finally, it will be shown how all of these individual improvements work together to provide
extended repair capability down to the 32 nm technology node.
Makers and users of advanced technology photomasks have seen increased difficulties with the removal of
persistent, or stubborn, nano-particle contamination. Shrinking pattern geometries, and new mask clean technologies
to minimize haze, have both increased the number of problems and loss of mask yield due to these non-removable
nano-particles. A novel technique (BitCleanTM) has been developed using a scanning probe microscope system
originally designed for nanomachining photomask defect repair. Progress in the technical development of this
approach into a manufacture-able solution is reviewed and its effectiveness is shown in selectively removing adherent
particles without touching surrounding sensitive structures. Methods for generating targeted edge test particles along
with considerations for removal of particles in various pattern geometries and materials are also discussed.
Improvements in repair process, software, and AFM tip technology have brought about an overall 2D shape
reconstruction capability to nanomachining that has not been previously imagined. Repair results are shown
for various processes to highlight their relative strengths and weaknesses. The impact of technical
improvements is shown in the advances in repair dimensional precision and overall imaging performance. The
greater technical potential of nanomachining is realized in this examination for mask repair scaled to smaller
repair geometries while repairing larger defects that may span these critical patterns.
It has been found that the femtosecond DUV laser mask repair tool has significant utility in the repair of unknown
foreign material (FM) contamination of sizes ranging from 50 nm to 30 μm with highly variable z-heights both
isolated and within critical complex patterns. Another significant ROI is the repair of masks which have already
been pelliclized (through pellicle repair or TPR) where the laser repair tool may work in conjunction with existing
through-pellicle inspection hardware to detect and remove FM and correct pattern errors. The capability of the tool
is also explored for repairs in patterns including the 45 nm technology node.
The transition to sub 45 nm technology nodes presents a significant technical challenge for mask repair due to a number of
previously lesser known physical phenomena. Nanomachining technology, which has a history of equally successful repair
of all photomask types-including Cr binary masks, has not been immune to these challenges. This has led to the
development of a number of distinctly new processes to meet these technical requirements. In one of the two new processes
reviewed in this work, the bulk of the defect is removed by applying compressive instead of tangential stresses to the
NanoBitTM during repair. This allows for 45 nm and smaller repairs, with sidewall angles and aspect ratios greater than 70°
and 2:1 respectively, in open mask structures. For repair in closed-or missing defect-structures, a process was developed to
minimize tip deflection away from the designated repair box boundaries for accurate two dimensional shape reconstructions
of deep and complex patterns. The successful application of this technique is shown for actinic phase-correct missing and
partial CPL, EAPSM, and Cr-absorber square contacts at these nodes. Additionally, the potential of these new processes to
enable higher aspect advanced NanoBitTM designs for robust mask repair, and the new processes developed to effectively
clean nanomachining debris from these advanced mask structures are discussed to provide a complete review of these
solutions and their supporting technologies.
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