EUV lithography is expected to be the most promising technology for semiconductor device manufacturing of the 7nm node and beyond. The EUV mask is a key element in the lithographic scanner optical path. The image border is a pattern free dark area around the die on the photomask serving as transition area between the parts of the mask that is shielded from the exposure light by the Reticle Masking (REMA) blades and the die. When printing a die at dense spacing on an EUV scanner, the reflection from the image border overlaps edges of neighboring dies, affecting CD and contrast in this area. This is related to the fact that EUV absorber stack reflects 1-3% of actinic EUV light. To reduce this effect several types of image border with reduced EUV light reflectance (<0.05%) have been proposed; such an image border is referred to as a black border (BB). In particular, an etched multilayer type black border was developed; it was demonstrated that CD impact at the edge of a die is strongly reduced with this type of the black border. However, wafer printing result still showed some CD change in the die influenced by the black border reflection. It was proven that the CD change was caused by DUV Out Of Band (OOB) light which is emitted from the EUV light source. In our previous study, a new types of multilayer etched BB called ‘Hybrid Black Border’ (HBB) had been developed and showed a good potential for DUV light suppression. OOB light reflection on HBB is ~3x lower than that of normal BB. Imaging performance was also demonstrated on NXE:3300 scanner system for N10 imaging structures of 16nm dense lines and 20nm isolated spaces. These results were compared to the imaging results obtained for a mask with the normal BB and 3x improvement was achieved; less than 0.2 nm CD changes were observed in the corners of the die. However, OOB light reflectance suppression was still not enough in short wavelength. In this study, we focused on OOB light reflectance reduction in short wavelength, and we developed a new HBB called ‘Advanced HBB’. We measured the OOB light reflectance of Advanced HBB by synchrotron radiation facility at PTB (Physikalisch- Technische Bundesanstalt, Germany). These results were compared to the results obtained from previous HBB. Then Advanced HBB achieved over 50% OOB light reflectance improvement in average wavelength 100nm to 270nm. Imaging performance also simulated in the edges and corners of the die. The CD-drop is expected to be more improved for Advanced HBB than previous HBB. As a result, it is expected the implementation of the Advanced HBB will help to mitigate the effects of possible increases of OOB light in the future higher power EUV sources.
EUV lithography is the most promising technology for semiconductor device manufacturing of the 10nm node and
beyond. The image border is a pattern free dark area around the die on the photomask serving as transition area between
the parts of the mask that is shielded from the exposure light by the Reticle Masking (REMA) blades and the die. When
printing a die at dense spacing on an EUV scanner, the reflection from the image border overlaps edges of neighboring
dies, affecting CD and contrast in this area. This is related to the fact that EUV absorber stack reflects 1-3% of actinic
EUV light. To reduce this effect several types of image border with reduced EUV reflectance (<0.05%) have been
proposed; such an image border is referred to as a black border. In particular, an etched multilayer type black border was
developed; it was demonstrated that CD impact at the edge of a die is strongly reduced with this type of the black border
(BB). However, wafer printing result still showed some CD change in the die influenced by the black border reflection. It
was proven that the CD shift was caused by DUV Out of Band (OOB) light from the EUV light source. New types of a
multilayer etched BB were evaluated and showed a good potential for DUV light suppression.
In this study, a novel BB called ‘Hybrid Black Border’ (HBB) has been developed to eliminate EUV and DUV OOB
light reflection by applying optical design technique and special micro-fabrication technique. A new test mask with HBB
is fabricated without any degradation of mask quality according to the result of CD performance in the main pattern,
defectivity and cleaning durability. The imaging performance for N10 imaging structures is demonstrated on
NXE:3300B in collaboration with ASML. This result is compared to the imaging results obtained for a mask with the
earlier developed BB, and HBB has achieved ~3x improvement; less than 0.2 nm CD changes are observed in the
corners of the die. A CD uniformity budget including impact of OOB light in the die edge area is evaluated which shows
that the OOB impact from HBB becomes comparable with other CDU contributors in this area. Finally, we state that
HBB is a promising technology allowing for CD control at die edges.
EUV lithography is the most promising technology for semiconductor device manufacturing of the 10nm node and beyond. The EUV mask is a key element in the lithographic scanner optical path. The image border is a pattern free dark area around the die on the photomask serving as transition area between the parts of the mask that is shielded from the exposure light by the Reticle Masking (REMA) blades and the die. When printing a die at dense spacing on an EUV scanner, the EUV light reflection from the image border overlaps edges of neighboring dies, affecting CD and contrast in this area. To reduce this effect an etched multilayer type black border was developed, and it was demonstrated that CD impact at the edge of a die is strongly reduced with this type of the black border (BB). However, wafer printing result still showed some CD change influenced by the black border reflection. It was proven that the CD shift was caused by DUV Out of Band (OOB) light which is emitted from EUV light source. New types of a multilayer etched BB were evaluated and showed a good potential for DUV light suppression. In this study, a novel black border called Hybrid Black Border has been developed which allows to eliminate EUV and DUV OOB light reflection. Direct measurements of OOB light from HBB and Normal BB are performed on NXE:3300B ASML EUV scanner; it is shown that HBB OOB reflection is 3x lower than that of Normal BB. Finally, we state that HBB is a promising technology allowing for CD control at die edges.
This paper discusses defectivity of a black border around the mask pattern of a reticle for extreme EUV lithography. An opaque image border is intended to overcome the limitation of the reticle masking blades of the scanner, in providing sufficiently sharp and accurate image delineation on wafer. The most commonly applied “black border” method for EUV reticles has the multilayer mirror removed in the image border area. A dedicated mask with such etched ML image border has been generated. It includes several modules of patterns, each surrounded by black border, so that each can be imaged separately with minimized background dose caused by its border. The printability of programmed defects within this image border has been assessed on an NXE3100 EUV scanner. Studied defect types include ML pedestals with and without absorber still on top. Especially the former must be totally avoided as such clear defect is very printable and can even erase parts of the pattern in neighboring dies.
Photomask is at the heart of a lithographic scanner’s optical path. It cannot be left non-optimized from the imaging point of view. In this work we provide new insights on two critical aspects of EUV mask architecture: optimization of absorber for 16 nm half-pitch imaging and a systematic approach to black border EUV and DUV reflectance specifications. Good 16 nm imaging is demonstrated on ASML NXE:3300 EUV scanner. Currently a relatively high dose resist is used for imaging and the dose reduction is desired. Optimization (reduction) of absorber height and mask CD bias can allow for up to 30% dose reduction without essential contrast loss. Disadvantages of absorber height reduction are ~7 nm increase of best focus range through pitch and tighter absorber height mean to target and uniformity requirements. A disadvantage of a smaller reticle CD (down to 14 nm 1x) is manufacturing process uniformity over the reticle. A systematic approach of black border reflections impact on imaging is established. The image border is a pattern free dark area surrounding the image field and preventing exposure of the image field neighborhood on wafer. Currently accepted design of the black border on EUV reticle is an image border where the absorber and multilayer stack are etched down to the substrate and EUV reflectance is reduced to <0.05%. DUV reflectance of such a black border is about 5%. It is shown that a tighter DUV reflectance specification <1.5% is required driven by the impact of DUV reflections from the black border on imaging. NXE:3300 and NXE:3100 experimental imaging results are shown. The need of low DUV wavelength reflectance metrology (in the range 100-300 nm) is demonstrated using an estimated NXE scanner out-of-band DUV spectrum. Promising results of low DUV reflectance of the black border are shown.
The impact of various mask parameters on CDU combined in a total mask budget is presented, for 22 nm lines, for reticles used for NXE:3300 qualification. Apart from the standard mask CD measurements, actinic spectrometry of multilayer is used to qualify reflectance uniformity over the image field; advanced 3D metrology is applied for absorber profile characterization including absorber height and side wall angle. The predicted mask impact on CDU is verified using actual exposure data collected on multiple NXE:3300 scanners. Mask 3D effects are addressed, manifesting themselves in best focus shifts for different structures exposed with off-axis illumination. Experimental NXE:3300 results for 16 nm dense lines and 20 nm (semi-)isolated spaces are shown: best focus range reaches 24 nm. A mitigation strategy by absorber height optimization is proposed based on experimental results of a special mask with varying absorber heights. Further development of a black image border for EUV mask is considered. The image border is a pattern free area surrounding image field preventing exposure the image field neighborhood on wafer. Normal EUV absorber is not suitable for this purpose as it has 1-3% EUV reflectance. A current solution is etching of ML down to substrate reducing EUV reflectance to <0.05%. A next step in the development of the black border is the reduction of DUV Out-of-Band reflectance (<1.5%) in order to cope with DUV light present in EUV scanners. Promising results achieved in this direction are shown.
To satisfy the requirement on the image placement accuracy, it is very important to consider the stress of the films on the mask substrate. The stress of the EUV mask is much larger than several kinds of optical masks because reflective Mo/Si multilayer (ML) has large compressive stress. In recent years, thinner absorber has been proposed because of better resolution and less shadowing effect. However it results in the leakage of the light to the adjacent chips on wafer. Then the light shield around the pattern area on the mask has been developed. From the viewpoint of manufacturability, etched multilayer black border (BB) is advantageous. Pattern displacement occurs at the edge of the multilayer etched BB. Measured pattern displacement error increased near the BB and it was simulated by using finite element method. The displacement depends strongly on the ML stress and it is successfully described by the release of the compressive stress at the edge of the black border. Most of the deformation near the BB remains even if the mask is chucked to the flat surface. Simulation using various models are discussed and compared with experimental results.
The image border is a pattern free dark area around the die on the photomask serving as transition area between
the parts of the mask that is shielded from the exposure light by the Reticle Masking (ReMa) blades and the die.
When printing a die at dense spacing on an EUV scanner, the reflection from its image border overlaps with the
edges of neighboring dies affecting CD and contrast in this area. This is related to the fact that EUV absorber
stack has 1-3% reflectance for actinic light. For a 55nm thick absorber the induced CD drop at the edges is
found to be 4-5 nm for 27 nm dense lines. In this work we will show an overview of the absorber reflection
impact on CD at the edge of the field across EUV scanner generations, for several imaging nodes and multiple
Increasing spacing between dies on the wafer would prevent the unwanted exposure but results in an
unacceptable loss of valuable wafer real estate thereby reducing the yield per wafer and is thus not a viable
manufacturing solution. In order to mitigate the reflection from the image border one needs to create a so called
black border. The most promising approach is removal of the absorber and the underlying multilayer down to
the low reflective LTEM substrate by multilayer etching. It was shown in the previous study that the impact
on CD was reduced essentially for 27 nm dense lines exposed on ASML NXE:3100.
In this work we will continue the study of a multilayer etched black border impact on imaging. In particular, 22
nm lines/spaces imaging on ASML NXE:3300 EUV scanner will be investigated in the areas close to the black
border as well as die to die effects. We will look closer into the CD uniformity impact by DUV Out-of-Band
light reflected from black border and its mitigation. A possible OPC approach will also be evaluated.
EUV lithography performance is improved significantly by optimizing and fine-tuning of the EUV mask. The EUV mask is an active element of the scanner optical system influencing main lithographic figure of merits such as image contrast, critical dimension uniformity (CDU), focus and overlay. The mask stack consists of Mo/Si multilayer acting as a bright field and a patterned absorber stack. In this work we will concentrate on investigation of EUV absorber. Absorber topography that is pronounced compared to the imaging wavelength of 13.5 nm, will give rise to various mask 3d effects such as shadowing or dependence of CD on feature orientation, best focus shift of different resolution structures, etc. Light interference in the absorber layer results in swinging behavior of various lithography metrics as function of the absorber height. Optimization of the mask absorber allows mitigating mask 3d effects and improving imaging performance. In particular, reduction of the absorber height mitigates the shadowing effect and relaxes requirements on Optical Proximity Correction (OPC), but can result in smaller Process Window due to lower imaging contrast and larger best focus shifts. In this work we will show results of an experimental approach to absorber height optimization. A special mask with 27 different absorber heights in the range 40-70 nm is manufactured by Toppan Photomasks. EUV reflectivity spectra are measured for the different absorber heights and an experimental swing curve is constructed. For each absorber height various resolution features are present on the mask. Lines of 27 nm and 22 nm are imaged on the wafer using the ASML EUV scanner NXE:3300B with an NA of 0.33. The experimental CD swing curve is constructed as well as HV change as a function of absorber height. The impact of the absorber height on Exposure Latitude (EL) and Dose to Size (D2S) is investigated. EL improves with increasing absorber height in some cases, however there is no clear EL gain for a 70 nm absorber compared to for example 52 nm absorber. D2S does show a clear trend through absorber height. In particular, D2S can be reduced by absorber height reduction: e.g. for 52 nm absorber D2S is 5% or 1 mJ/cm2 smaller compared to 70 nm. The experimental results are used for calibration and verification of rigorous mask 3d simulations. This knowledge is crucial for accurate OPC of production masks and allows for accurate litho simulations of EUV user cases as a basis for lithography roadmaps towards High Volume Manufacturing and High NA EUV.
There are multiple mask parameters that can be tuned to optimize the lithographic performance of the EUV
photo mask. One of them is the absorber height. A reduction of the absorber height allows, for example, a
higher resolution patterning on mask and reduces the OPC needed for shadowing correction. Downside of
a thinner absorber is the increased reflectivity which manifests itself not only in the image field (contrast loss)
but also in the so called light shield area or image border.
The image border is a pattern free (absorber covered) area around the die on the photo mask forming the
transition area between the part on the mask that is completely shielded from the exposure light by the Reticle
Masking (REMA) blades and the die. The image border accommodates the finite REMA placement accuracy
and the half shadow of the REMA blades allowing close spaced die printing on the wafer.
When printing a die at dense spacing, which is common practice in a production environment, the image border
will overlap part of the neighboring die. This causes actinic EUV and DUV out of band light reflection from the
image border exposing the overlapped die area and affecting CD and contrast at the edges of the dies. For a 44
nm thick absorber we found a CD impact of 8 nm for 32 nm dense lines whereas for a 55 nm thick absorber
the effect was 4 nm for 27 nm dense lines. Increasing the die spacing would prevent this unwanted exposure
but results in an unacceptable loss of valuable wafer real estate thereby reducing the yield per wafer and is thus
not a viable manufacturing solution.
Optical Proximity Correction (OPC) using ASML Brion’s Tachyon NXE model at the edges of the die was
proposed as possible solution to this problem. An alternative is to create a so called Black Border: the
reflectivity in the image border is reduced to a sufficiently low level by for example increasing the absorber
thickness, add a special coating or replace the absorber with a low reflective material. The most radical
solution is removal of the absorber and the underlying multilayer down to the low reflective substrate, so-called
In this paper we will present the effects of such a Black Border created by a multilayer etch on features and
their placement on the reticle and the impact on CD of 27 nm dense lines on the wafer. By comparing the wafer
CDU printed with and without Black Border we will determine how well the image border effect is mitigated by
the multilayer etching.
EUV lithography is the most promising candidate for semiconductor device manufacturing of 1x nm half pitch and
beyond. For the practical use, EUV mask with a thin absorber could be adopted because of less shadowing effect. EUV
reflectivity from the thin absorber is about 1~3%. It would cause CD change on wafer especially at the exposure field
edge due to the leakage of the EUV light from neighboring exposure shots.<sup>1</sup> To avoid this phenomenon, light shield
black border is needed at the edge of pattern area on mask. Stacked absorber type and ML-etched type of light shield
black border have been proposed in the past.<sup>2</sup> The most important things for these black borders are that there is no
reflection of EUV light and no defect which affects pattern CD on wafer. ML-etched black border is considered to be
applied for early practical use from a viewpoint of manufacturability. Because CD degradation and defect increase might
happen due to 2<sup>nd</sup> litho and etch process on its main pattern area in manufacturing process of stacked absorber type.
In this paper, we will show several evaluation results regarding
ML-etched black border we have developed. It has a
good light shield performance for EUV and low DUV light reflection. Defect inspection in black border area can be
performed successfully by three kinds of inspection tools. As a result, most of the defects seemed not to be printable to
wafer. We also evaluated CD change, flatness change linked to mask IP shift and particle contamination on main pattern
area. What it comes down to is that there is no show-stopper for
ML-etched BB process for now.
The miniaturization of pattern size on photomask is advanced year by year. It becomes more important to improve Line
Edge Roughness (LER) and resolution because of their impacts on lithography performances. When miniaturization is
advanced, high sensitivity inspection is also indispensable. Therefore, LER becomes the key factor to reduce the
nuisance defect for high sensitivity mask inspection. Basically, LER originates from resist materials and EB writer. If
resist pattern LER is good, final pattern LER can be good too. One of the easiest solutions for LER is using thick resist.
Thick resist can vertically smooth down the LER. However, it deteriorates resolution due to the high aspect-ratio.
Another solution for LER is using low sensitivity resist. Low sensitivity resist needs many electron exposures by EB
writer. Therefore, electronic density of EB pattern increases and pattern edge becomes clear. However, it deteriorates
throughput, which is essential to production. Only by mask resist, it is difficult to satisfy all items, that is mask LER,
resolution and throughput.
In this study, the improvement of LER without deterioration of resolution is tried by dry etching process. It is found that
remaining resist after Cr etching has its limitation for mask LER. And Cr over etching and source power of Cr and MoSi
etching are effective factors for mask LER. On the basis of these results, the optimal etching process is determined. It is
confirmed that mask LER can be improved without deterioration of resolution by the optimal etching process.
We developed a pattern shape analysis tool (MaskEXPRESS) which can evaluate quantitatively photomask pattern and fabrication process by means of image processing arising from CD-SEM or UV microscope, or inspection machine. Although evaluation of complicated mask pattern has been performed qualitatively as yet, MaskEXPRESS makes it possible to evaluate it quantitatively. MaskEXPRESS can also be applied to quantitative evaluation of sensitivity of inspection machine, accuracy of EB writing, and optimization of photomask fabrication process. This paper describes the outline of MaskEXPRESS and its functions. We investigated about the precision criteria of MaskEXPRESS and found out the conditions of image processing for having accuracy equal to repeatability accuracy of measurement SEM. By changing experimentally mask fabrication conditions and analyzing the patterns, the following things became clear. Hole pattern's area increase with keeping analogous shape as etching time increases. Inner serif pattern tends to change in the direction of slant as writing dose increases. The rectangle fidelity of inner and outer serif pattern is improved according to the condition of resist process. We also present the relationship between defect size and aerial image on wafer simulated utilizing MaskEXPRESS.
It has been used to measure the maximum length of defect size for the defect decision method at the reticle inspection review. But since 0.25-0.18 micrometers node, we need to have another method to measure and judge the defect because of the complicated pattern line OPC shape and defects which could not decide to be acceptable or not for sensitive defect printability. The best way to know the effect of defects is to print on wafer or to use special review tool so called optical lithography simulation microscope like AIMS in order to judge these defects. But AIMS requires optical parameter of the wafer exposure machine. And its operation takes much time. And most of the detected defects can be judged at the photomask inspection process. We propose new judgement method for defect review precisely and easily. We have developed pattern shape analysis tool that makes defect shape of inspection review image some contact hole pattern example measured by its area and intensity values or another image acquisition system like SEM some quantitative expression. This method is useful for measuring the defect on a complicated pattern like OPC, corner rounding or edge roughness as pattern quality, or area size of a contact hole. Moreover, this method does not remain at the measurement with 2D pattern and can take the total quality of the light as the flux as well. We measured the shape of the mask pattern and the defect quantitatively using this method and evaluated print possibility about the defect print step.