At the most advanced technology nodes, such as 32nm and 22nm, aggressive OPC and Sub-Resolution Assist Features
(SRAFs) are required. However, their use results in significantly increased mask complexity, making mask defect
disposition more challenging than ever. This paper describes how mask patterns can first be recovered from the
inspection images by applying patented algorithms using Level Set Methods. The mask pattern recovery step is then
followed by aerial/wafer image simulation, the results of which can be plugged into an automated mask defect
disposition system based on aerial/wafer image. The disposition criteria are primarily based on wafer-plane CD variance.
The system also connects to a post-OPC lithography verification tool that can provide gauges and CD specs, thereby
enabling them to be used in mask defect disposition as well. Results on both programmed defects and production defects
collected at Samsung mask shop are presented to show the accuracy and consistency of using the Level Set Methods and
aerial/wafer image based automated mask disposition.
A new methodology - Aerial Plane Inspection (API) - has been developed to inspect advanced photomasks
used for the 45 nm node and beyond. Utilizing images from a high resolution mask inspection system, a
mask image is recovered by combining the transmitted and reflected images. A software transformation is
then performed to replicate the aerial image planes produced in a photolithography exposure system. These
aerial images are used to compare adjacent die in a Die-Die inspection mode in order to find critical defects
on the photomask. The mask recovery process and modeling of the aerial plane image allows flexibility to
simulate a wide range of lithographic exposure systems, including immersion lithography. Any source
shape, Sigma, and numerical aperture (NA) can be used at all common lithographic wavelengths.
Sensitivity of the inspection can be fully adjusted to match photomask specifications for CD control, lineend
shortening, OPC features, and for small and large defective areas. An additional adaptive sensitivity
option can be utilized to automatically adjust sensitivity as a function of MEEF.
Using the Aerial Plane Inspection to compare pattern images has the benefit of filtering out non-printing
defects, while detecting very small printing defects. In addition, defects that are not printing at ideal
exposure condition, but may be reducing the lithographic process window, can also be detected.
Performing defect detection at the aerial image plane is more tolerant to small Optical Proximity Correction
(OPC) sub-resolution assist features (SRAFs) that are difficult to inspect at the reticle image plane.
In the ever-changing semi-conductor industry, new innovations and technical advances constantly bring new
challenges to fabs, mask-shops and vendors. One of such advances is an aggressive optical proximity
correction (OPC) method, sub-resolution assist features (SRAF). On one hand, SRAFs bring a leap forward
in resolution improvement during wafer printing; on the other hand they bring new challenges to many
processes in mask making. KLA-Tencor Corp. working together with Samsung Electronics Co. developed an
additional function to the current HiRes 1 detector to increase inspectability and usable sensitivity during the
inspection step of the mask making process. SRAFs bring an unique challenge to the mask inspection process,
which mask shops had not experienced before. SRAF by nature do not resolve on wafer and thus have a
higher tolerance in the CD (critical dimension) uniformity, edge roughness and pattern defects.
This new function, Thin-Line De-sense (TLD), increase the inspectability and usable sensitivity by generating
different regions of sensitivity and thus will match the defect requirement on a particular photomask with
SRAFs better. The value of TLD was proven in a production setting with more than 30 masks inspected, and
resulted in higher sensitivity on main features and a sharp decrease in the amount of defects that needed to be
classified.
As design rules continue to shrink towards 4x nm, there are increase usage of aggressive Optical Proximity Correction
(OPC) in reticle manufacturing. One of the most challenging aggressive OPCs is Sub Resolution Assist Feature (SRAF)
such as scattering and anti-scattering bars typically used to overlap isolated and dense feature process windows. These
SRAF features are sub-resolution in that these features intentionally do not resolve on the printed wafer. Many reticle
manufacturers struggle to write these SRAFs with consistent edge quality even the most advanced E-Beam writers and
processes due to resolution limitations. Consequently, this inconsistent writing gives reticle inspection
challenges. Large numbers of such nuisance defects can dominate the inspection and impose an extraordinarily high
burden on the operator reviewing these defects. One method to work around inconsistent assist feature edge quality or
line-end shortening is to adjust the mask inspection system so that there is a substantial sensitivity decrease in order to
achieve good inspectability, which then compromises the sensitivity for the defects on main geometries.
Modern defect inspection tools offer multiple modes of operation that can be effectively applied to optimize defect
sensitivity in the presence of SRAF feature variability. This paper presents the results of an evaluation of advance
inspection methods and modes such as die to database selective thinline desense, transmitted & reflected light
inspections, review system and die to die selective desense to increase inspectability and usable sensitivity using
challenging production and R&D masks.
Key learnings are discussed.
The importance of mask pattern inspection is increased as design node shrinks below. The major reason is as follows.
Firstly, inspection systems have to enhance sensitivity because the high grade devices are seriously affected from small
defects compared with low grades. The other is SRAFs RET masks. In order to inspect SRAF properly, inspection
systems need severer conditions such as small pixel size, short wavelength and special algorithms. Therefore, it takes
more than 3 hours to inspect a mask and this increasing inspection time is a serious burden in mask making process.
Moreover in spite of mask market and its infrastructure, cost of inspection system is too high.
In this paper, the advantages of using Xe-Hg lamp instead of a DUV laser are presented. Special defect algorithms get
over low sensitivity of lamp optics. We have evaluated performances of the defect inspection system with programmed
defect mask and production mask.
The inspection system is cost-effective because the optic part is configured by DUV lamp and fiber optic delivery
system. The fast scanning speed is enough to charge the inspection capacity in the fabrication line. These features of the
system well match with the flexibility of the facility layout in the mask production.
Mask inspection plays a pivotal role in current high grade mask making processes and further its importance is getting bigger. The purpose of inspection process is as follows. One is simple sorting of NG masks that have fatal defects with high sensitivity. The other is improvement of total mask manufacturing process and mask quality using defect source analysis. As semiconductor device is getting shrunk down, the influence of mask defect is increasing. Therefore, there are special needs for the efficient use of such expensive inspection machines and the systematic approach of defect analysis. In this paper, we propose novel mask inspection flow to improve mask inspection capacity and systematic defect management. In general, Inspection process is divided by two steps. One is detection of defects and the other is review for defect analysis. Our concept of new inspection flow is adoption of individual defect review system after defect detection in inspection machine.
With this new inspection flow using defect review system, we could increase inspection capacity by 30% and set up unified defect analysis hub.
RET (Resolution Enhancement Technique) is strongly required for 65nm node pattern generation. Alternating Phase Shift Masks (APSM) and Chrome-less Phase Lithography (CPL) masks are widely used for the purpose of RET. However, APSM and CPL mask manufacturing is rather complex and difficult in terms of their structure and fabrication. To inspect these kind of RET masks is very difficult because of quartz (Qz) phase defects which can hardly be detected by using a conventional inspection method. Since Qz phase defect is the key issue in APSM or CPL mask manufacturing, many works have been done widely so far. Here we've evaluated the defocus inspection method to find best inspection condition for detecting Qz phase defects. We conclude that the best condition for finding Qz phase defects could have dependency upon the pattern shape and size. Moreover, the limitation of the inspection capability for Qz phase defect inspection has been addressed with comparison of the wafer print result.
As the design rule continues to shrink towards 65nm size and beyond the defect criteria are becoming ever more challenging. Pattern fidelity and reticle defects that were once considered as insignificant or nuisance are now becoming significant yield impacting defects. The intent of this study is to utilize the new generation DUV system to compare Die-to-Die Reflected Light inspection and Die-to-Die Transmitted Light Inspection to increase defect detection for optimization of the 65nm node process.
In addition, the ReviewSmart will be implemented to help categorically identify systematic tool and process variations and thus allowing user to expedite the learning process to develop a production worthy 65nm node mask process. The learning will be applied to Samsung's pattern inspection strategy, complementing Transmitted Light Inspection, on critical layers of 65 nm node to gain ability to find defects that adversely affect process window.
Defect is a killing factor in photomask fabrications. For 65nm node photomask fabrication, even smaller than 1 um particle can cause hard-to-repair defect. And it is not easy to find the defect source and solve it. For this reason, the process monitoring system that shows us current defect trend rapidly and effectively is highly required. At the same time, this system can be used for verifying the process stability and detecting unusual signals in process.
The extension of KrF lithography has become the major trend in semiconductor manufacturing due to the delay of ArF lithography. Therefore, various resolution enhancement techniques (RETS) are employed for sub 100nm node patterning. This paper introduces the 100 percent transmission PSM as a candidate for resolving the problems with previous approaches using a transparent phase-shift mask. CLM shows a high optical performance and relatively simple mask fabrication compare to other strong phase-shift mask. However, full-chip level CLM application is still under development due to the difficulty of mask manufacturing and lack of proper layout converting environment. In this paper, we covered mask-making process such as the quartz dry etch and defect engineering which are critical to CLM manufacturing. We made a test mask based on the basic CLM concept and evaluated its optical performance. Finally, we will show the feasibility of chrome-less mask manufacturing for real device application.
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