As the mask specifications become tighter for low k1 lithography, more aggressive repair accuracy is required below sub 20nm tech. node. To meet tight defect specifications, many maskshops select effective repair tools according to defect types. Normally, pattern defects are repaired by the e-beam repair tool and soft defects such as particles are repaired by the nanomachining tool. It is difficult for an e-beam repair tool to remove particle defects because it uses chemical reaction between gas and electron, and a nanomachining tool, which uses physical reaction between a nano-tip and defects, cannot be applied for repairing clear defects.
Generally, film deposition process is widely used for repairing clear defects. However, the deposited film has weak cleaning durability, so it is easily removed by accumulated cleaning process. Although the deposited film is strongly attached on MoSiN(or Qz) film, the adhesive strength between deposited Cr film and MoSiN(or Qz) film becomes weaker and weaker by the accumulated energy when masks are exposed in a scanner tool due to the different coefficient of thermal expansion of each materials. Therefore, whenever a re-pellicle process is needed to a mask, all deposited repair points have to be confirmed whether those deposition film are damaged or not. And if a deposition point is damaged, repair process is needed again. This process causes longer and more complex process.
In this paper, the basic theory and the principle are introduced to recover clear defects by using nanomachining tool, and the evaluated results are reviewed at dense line (L/S) patterns and contact hole (C/H) patterns. Also, the results using a nanomachining were compared with those using an e-beam repair tool, including the cleaning durability evaluated by the accumulated cleaning process. Besides, we discuss the phase shift issue and the solution about the image placement error caused by phase error.
As the number of masks per wafer product set is increasing and low k1 lithography requires tight mask
specifications, the patterning process below sub 20nm tech. node for critical layers will be much more expensive
compared with previous tech. generations. Besides, the improved resolution and the zero defect level are necessary to
meet tighter specifications on a mask and these resulted in the increased the blank mask price as well as the mask
Unfortunately, in spite of expensive price of blank masks, the certain number of defects on the blank mask is
transformed into the mask defects and its ratio is increased. But using high quality blank mask is not a good idea to
avoid defects on the blank mask because the price of a blank mask is proportional to specifications related to defect
level. Furthermore, particular defects generated from the specific process during manufacturing a blank mask are
detected as a smaller defect than real size by blank inspection tools because of its physical properties. As a result, it is
almost impossible to prevent defects caused by blank masks during the mask manufacturing.
In this paper, blank defect types which is evolved into mask defects and its unique characteristics are observed.
Also, the repair issues are reviewed such as the pattern damage according to the defect types and the repair solution is
suggested to satisfy the AIMS (Arial Image Measurement System) specification using a nanomachining tool.
Main Topics of a photomask have been CD(Critical Dimension), Overlay and Defects. In side of
defects, technique suppressing growing defects which are occurring on a mask surface becomes as
important as defect control method during mask manufacturing process. Conventional growing defects
arise out of combination of sulfuric ion on a mask surface and environmental facts such as pellicle
ingredient, humidity and etc. So Mask cleaning process was driven to reduce sulfuric acid on a mask
surface which source of growing defects. And actually various cleaning process has been developed
through the elimination of sulfuric acid such as DI, O3 cleaning. Normally Conventional growing defects
are removed using DI, SC1 or SPM cleaning according to incidence.
But recently irregular growing defects are occurred which are completely distinct from conventional
growing defects. Interestingly, irregular growing defects are distributed differently from conventional on a
mask. They spread in isolated space patterns and reduce the transmittance so that space pattern size
continuously decreased. It causes Wafer Yield loss. Furthermore, irregular growing defects are not fully
removed by cleaning which is traditional removal process. In this study, we provide difference between
conventional and irregular growing defects based on its characteristic and distributed formation.
In addition, we present and discuss removal and control technique about irregular growing defects. For
elemental analysis and study, diverse analysis tool was applied such as TEM for checking Cross-Section,
AFM for checking the roughness of surface, EDAX, AES, IC for analyzing remained ions and particles on
the mask and AIMS.
As design rule of memory device is smaller and smaller, the CD uniformity of a photomask become the most
important factor to satisfy wafer exposure performance. Once the photomask is made, CD uniformity of the mask
can't be changed and if CD uniformity of the mask is not good to use for wafer exposure, we must reject it and
make another one again. But, after applying transmission control tool for CD uniformity, we have an extra chance
to control mask CD uniformity in one mask and this is very effective for wafer printing result.
In this paper, we are going to evaluate the behavior of wafer CD due to transmission control position change
within photomask substrate and find the optimum control position for better wafer result.
Since mask design rule is smaller and smaller, Defects become one of the issues dropping the mask yield.
Furthermore controlled defect size become smaller while masks are manufactured. According to ITRS roadmap on
2007, controlled defect size is 46nm in 57nm node and 36nm in 45nm node on a mask. However the machine
development is delayed in contrast with the speed of the photolithography development.
Generally mask manufacturing process is divided into 3 parts. First part is patterning on a mask and second part is
inspecting the pattern and repairing the defect on the mask. At that time, inspection tools of transmitted light type are
normally used and are the most trustful as progressive type in the developed inspection tools until now. Final part is
shipping the mask after the qualifying the issue points and weak points. Issue points on a mask are qualified by using
the AIMS (Aerial image measurement system).
But this system is including the inherent error possibility, which is AIMS measures the issue points based on the
inspection results. It means defects printed on a wafer are over the specific size detected by inspection tools and the
inspection tool detects the almost defects. Even though there are no tools to detect the 46nm and 36nm defects
suggested by ITRS roadmap, this assumption is applied to manufacturing the 57nm and 45nm device.
So we make the programmed defect mask consisted with various defect type such as spot, clear extension, dark
extension and CD variation on L/S(line and space), C/H(contact hole) and Active pattern in 55nm and 45nm node. And
the programmed defect mask was inspected by using the inspection tool of transmitted light type and was measured by
using AIMS 45-193i. Then the marginal defects were compared between the inspection tool and AIMS. Accordingly we
could verify whether defect size is proper or not, which was suggested to be controlled on a mask by ITRS roadmap.
Also this result could suggest appropriate inspection tools for next generation device among the inspection tools of
transmitted light type, reflected light type and aerial image type.
The Aerial Image Measurement Tool (AIMS) can estimate the wafer printability without exposure to wafer by using
scanner. Since measured aerial images are similar with wafer prints, using AIMS becomes normal for verifying issue
points of a mask. Also because mask design rule continues to shrink, defects and CD uniformity are at issues as factors
decreasing mask yield. Occurred defects on a mask are removed by existing mask repair techniques such as
nanomachining, electron beam and focused ion beam. But damages and contaminants by chemical and physical action
are found on the mask surface and contaminants above special size lead to defects on a wafer. So cleaning has been
necessary after repair process and detergency has been important. Before AIMS measurement, cleaning is done to make
same condition with shipped mask, which method brings repeated process - repair and cleaning - if aerial image was
So cleaning effect after the FIB repair is tested by using the AIMS to find the optimized process minimizing the
repeated process and to get similar scanner results. First, programmed defect mask that includes various defect size and
type is manufactured on some kinds of patterns in DRAM device and sub-80nm tech. Next the defects on the
programmed mask are repaired by FIB repair machine. And aerial images are compared after the chemical cleaning,
non-chemical cleaning and without cleaning.
Finally, approximate aerial images to scanner results are taken regardless of cleaning process. It means that residue
originated from repair process doesn't affect aerial images and flexible process is possible between AIMS, repair and
cleaning process. But as the effect of minute particles and contaminations will be increased if pattern size is much
smaller, it needs to reconfirm the effect below the sub-60nm in DRAM device.
The AIMS (Aerial Image Measurement Tool) measures approximate aerial images to scanner results by adjusting the
numerical aperture, illumination type and partial parameters. Accordingly, AIMS tool is used generally to verify the
issue points during manufacturing a mask. Normally using a mask for photolithography needs twice verifications. One
is the qualification in the mask shop. The other is verification over the photo process using the mask in the wafer fab. If
evaluated data at AIMS can be trusted about photo process ability including energy latitude (EL), depth of focus (DOF),
CD uniformity (CDU), pattern fidelity and mask defects including repair area, AIMS can function as a first filter before
shipping the mask. That means the AIMS data can be used as a preliminary data in the wafer fab.
So this study is focused on correlation between measured data at AIMS fab 193i and ArF scanner over the photo
process such as EL, DOF, CDU, pattern fidelity and mask defects. First, various patterns are made on attenuated PSM
from 80 to 65nm tech. Next correlations are calculated about EL, DOF and CDU by using same optical conditions,
measurement points and etc at AIMS and Scanner. Also the aerial images from AIMS are compared with scanner results
on defective side how those are matched with each other.
Consequently defect printability and CDU map at AIMS were similar to the scanner. In CDU point of view, AIMS
exceeds the predictive ability of the mask CD SEM. Moreover it means that wafer CDU can be corrected (improved)
independently on the CDU result of the wafer fab by using CDU correctable femto laser tool which reduces
transmittance of the mask. Surprisingly, it is possible. And Aerial image about mask defects including repair area is
useful to predict the problem of the mask, since it is similar to wafer results. But aerial image compared with wafer
image has more difference at 65nm technology node than at 80nm. If adjustment of threshold or measuring method can
be done, prediction of the scanner result will have no matter. In conclusion, predictive results at AIMS over photo
process can be applied as a preliminary data and it can be used to another index verifying the mask quality.
As mask feature size is shrinking, required accuracy and repeatability of mask CD measurement is more severe. CD-SEM which is usually used to measure below 0.5um pattern shows the degradation of repeatability by the sparkle noise. To reduce this, larger ROI (range of interest) is recommended on line and space patterns. But this wide ROI is difficult to use on Hole or isolated patterns. In this paper, anisotropic diffusion filtering method will be introduced to replace the ROI, and evaluated on various patterns such as holes and isolated patterns. It can also reduce the effects of defocus of CD-SEM and enhance the repeatability of CD-SEM. And multi-point CD measurement technique is described to reduce the local CD errors on CD uniformity of mask which is usual on one dimensional CD measurement conventionally. Using these methods, local CD uniformity and global CD uniformity of masks which is the key performance of mask quality can be measured more exactly compared to old CD measurement method. And we can give correct information of mask to reduce global CD uniformity by process tuning such as FEC (Fogging Effect Correction) or development process.