As the industry embarks on sub 50nm half pitch design nodes, higher resolution and advanced
photomask inspection algorithm are needed to resolve shrinking features and find critical yield limiting
defects. In this paper, we evaluate the detection capability of STARlight2+ 72nm pixel on sub-50nm
memory masks.
The mask sets targeted for this evaluation were focused on critical layers. Although memory mask
sets are dominated by multi-die layout, single die layout masks were also inspected because of their
significance during research and development. Inspection results demonstrated the performance of
STARlight2+ based on its sensitivity to contamination defects and the inspectability of masks with this
detection method. The most common plan of record for mask inspection in a wafer fab is die-to-die
transmitted pattern inspection modes, which limits the inspection area to the die region only and cannot be
used for single-die reticle inspections. However, STARlight2+ has single die inspection capability, which
is also needed in order to inspect scribe-lines and frame areas.
The primary defects of interest are photo induced crystal defects or haze. Haze continues to be the
primary reason for mask returns at 193nm exposure across the industry. The objective of this paper is to
demonstrate STARlight2+ 72nm capability to support memory wafer fab mask qualification requirements.
As the industry embarks on sub 50nm half pitch design nodes, higher resolution and advanced
inspection algorithm are needed to resolve shrinking features and find critical yield limited defects. In
this paper, we evaluate the detection capability of STARlight2+ 72nm pixel on DRAM masks.
The mask sets targeted for this evaluation were focused on critical layers. Although memory
mask sets are dominated by multi-die layout, single die layout masks were also inspected because of
their significance during research and development. Inspection results demonstrated the performance of
STARlight2+ based on its sensitivity to contamination defects, inspectability, first time success rate and
throughput. STARlight2+ has single die inspection capability, which is also needed in order to inspect
scribe-lines and frame areas.
The primary defects of interest are photo induced defects or contamination, causing mask
degradation. Contamination continues to be the primary reason for mask returns at 193nm exposure
across the industry. The objective of this paper is to demonstrate STARlight2+ 72nm capability to
support memory wafer fab mask qualification requirements.
At Photomask Japan 2007 the new algorithm of Fast Integrated die-to-die T+R (DDTR) for the views of P90 and P72
for the KLA Tencor TeraScanHR mask inspection system was presented. At the same time a new algorithm for P72 in
database tritone mode for reflected light (DBRt) was introduced. Both modes can be used together as one combined
inspection to detect pattern and contamination defects on production masks.
It was shown that these new algorithms allow for creation of a new inspection strategy with improved throughput and a
reduced amount of inspections. Currently an inspection strategy has to cover at first a pattern inspection (normally a
combination of die-to-die and additional database inspections) for finding hard defects on a mask and then as second step
a contamination inspection (STARlight2TM). The hard defects have to be repaired and the contaminations can be cleaned.
The new inspection strategy allows for detection of all critical hard and contamination defects on a mask with one single
combined inspection, enhancing productivity.
At BACUS 2007 the first evaluation of this new kind of inspection strategy for manufacturing of masks was
described for two production plates of different design. At that time only the database reflected tritone algorithm for the
view of P72 was available. The changes in inspection strategy could only go together with a change of view from P90 to
P72.
With view P72 higher overall sensitivity could be reached and smaller secondary features could be inspected. However,
these improvements may not be necessary for all plates and may need more time than a comparable P90 inspection.
Today the standard contamination inspection for critical masks is the P90 STARlight2TM (SL2). To do a time effective
parallel combo inspection with DDTR and DBRt the same view has to be used.
An extension of the database reflected tritone algorithm to the P90 view is now available. This gives the mask
manufacturer the flexibility to change the inspection strategy for P90 and P72 dependent only on feature size on mask or
minimal allowed defect size.
The results of the evaluation and the comparison of this P90 database reflected tritone algorithm with the P90
STARlight2TM and P90 Fast Integrated die-to-die T+R will be presented in this paper. It will be shown that comparable
results can be expected for P90 DBRt and P90 DDTR versus P90 STARlight2TM without missing any critical defects.
'Fast Integrated Die-to-Die T+R' pattern inspection (DDTR), reflected tritone database inspection (DBRt) and
STARlight2TM (SL2) contamination inspection are employed by mask makers in order to detect pattern defects and
contamination defects on photomasks for in process inspection steps.
In this paper we compare the detection capabilities of these modes on real production masks with a representative set of
contamination and pattern defects.
Currently, SL2 inspection is used to find contamination defects and die-to-die and die-to-database are used for pattern
defects. In this paper we will show that the new introduced 'Fast Integrated Die-to-Die T+R' pattern inspection
(DDTR)1 in combination with the DBRt can be used in production environment, instead of SL2 without any loss in the
sensitivity.
During the study, we collected and analyzed inspection data on critical layers such as lines & spaces and contact holes.
Besides, performance of the modes on product plates characterization was done using a test mask with programmed
defects.
Transmitted Light (ddT or dbT) pattern inspection and STARlight-2TM (SL2) contamination inspection are widely
employed by mask makers in order to detect pattern and contamination defects on photomasks during the mask
inspection process. However, such an approach needs a two-pass inspection to detect pattern defects and contamination
defects separately.
In this paper we introduce the 'Fast Integrated T+R and SL2' capability and investigate the properties of this
combination of Transmitted (T) and Reflected (R) light inspection on die areas and STARlight-2TM(SL2) on scribe
areas. 'Fast Integrated T+R and SL2' has the capability to reduce a two-pass inspection to a single set-up and single
pass inspection resulting in a substantial saving of inspection time. In addition to a throughput enhancement, 'Fast
Integrated T+R and SL2' is able to compliment the pattern T inspection by providing additional sensitivity to detect
challenging defects.
During this study we collect and analyze inspection data on a critical layer provided by the Advanced Mask Technology
Center. Compared to the 2-pass individual mode pattern T and contamination SL2 inspections, a single scan 'Fast
Integrated T+R and SL2' demonstrates the capability to capture additional real defects, improves reticle inspectability
and first time success rate, and results in a significant enhancement in productivity.
Based on empirical data collected in this study, the Fast Integrated T+R and SL2 inspection is able to improve inspection
throughput approximately 45% at P90.
High resolution mask inspection in advanced wafer fabs is a necessity. Initial and progressive mask defect problem still remains an industry wide mask reliability issue. Defect incidences and its criticality vary significantly among the type of masks, technology node and layer, fab environment and mask usage. A usage and layer based qualification strategy for masks in production need to be adopted in wafer fabs.
With the help of a high-resolution direct reticle inspection, early detection of critical and also non-critical defects at high capture rates is possible. A high-resolution inspection that is capable of providing necessary sensitivity to critical emerging defects (near edge) is very important in advanced nodes. At the same time, a way to disposition (make a go / no-go decision) on these defective masks is also very important. As the impact of these defects will depend on not only their size, but also on their transmission and MEEF, various defect types and characteristics have to be considered.
In this technical report the adoption of such a high-resolution mask inspection system in wafer fab production is presented and discussed. Data on this work will include inspection results from advanced masks, layer and product based inspection pixel assignment, defect disposition and overall wafer fab strategies in day-to-day production towards mask inspection.
Mask inspection and qualification is a must for wafer fabs to ensure and guarantee high and stable yields. Single defect events can easily cause a million dollar loss through a defect duplicating onto the wafer. Several techniques and methods for mask qualification within a wafer fab are known but not all of them are neither used nor understood regarding their limitations. Increasing effort on existing tool platforms is necessary to detect the defects of interest which are at the limit of the tools specification - On the other hand next generation tools are very sensitive and therefore consume only a negligible amount of time for recipe optimization. Knowing the limits of each inspection tool helps to balance between effort and benefit. Masks with programmed defects of 90nm and 65nm design rule were used in order to compare the different available inspection techniques. During the course of this technical work, the authors concentrate mainly on two inspection techniques. The first one inspects the reticle itself using KLA-Tencor's SLF27 (TeraStar) and SL536 (TeraScan) tools. As the reticle gets inspected itself this is the so called "direct" mask defect inspection. The second inspection technique discussed is the "indirect" mask defect inspection which consists of printing the pattern on a blank wafer and use KLA-Tencor's bright-field wafer inspection tool (2xxx series) to inspect the wafer. Data of this work will include description of the techniques, inspection results, defect maps, sensitivity analysis, effort estimation as well as limitations for both techniques for the used design rule.
Defect inspection of Quartz-PSMs is challenging, as the optical contrast for defects within the quartz substrate is small. The performance of three phase contrast algorithms is studied with a variety of defect test masks. For alternating phase shift masks key parameters such as optimum focus offset, defect sensitivity for different feature sizes, as well as defect sensitivity with respect to defect printability criteria are studied. In the studied range for two of the algorithms the defect sensitivity is independent of the feature size, whereas the third algorithm exhibited a decrease in sensitivity with decreasing feature sizes. In focus runs performed on large feature sizes a single optimum focus offset is observed, whereas for small feature sizes a two-path inspection using a positive and negative focus offset is found necessary. With respect to defect printability all critical 180° defects were found. For the newest of the three algorithms good inspectability of chrome-less PSMs is achieved.
Alternating phase shift masks (altPSMs) are a promising resolution enhancement technique to realize smaller design rules at the same lithography wavelength. Quartz defect inspection of altPSMs is challenging, as the optical contrast for defects within the quartz substrate is very small. AltPSM inspection capability was studied with different types of programmed test masks. The programmed quartz defects were characterized with a scanning electron microscope, an atomic force microscope and an aerial imaging microscope system. Finally a defect printability study was done. With the programmed test masks the performance of two altPSM inspection techniques was evaluated. Quartz defect detection was studied with respect to different pattern types and sizes. Quartz defect sensitivity was measured with respect to defect size as well as defect printability. It was found that quartz defect sensitivity with respect to defect size is constant for different pattern types, but decreases for decreasing line widths on 1:1 pitch line and space patterns. Whereas defect detection for the altPSM algorithms studied is governed by the defect’s extension perpendicular to the pattern line, defect printability is determined by the defect’s lateral area.
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