For many years, the preservation of Moore’s law has been achieved by the means of Multi Pattering Technologies (MP) and Immersion lithography. Starting from the 7nm technology node, the Era of EUV mass production started to play a fundamental role in semiconductor manufacturing industry. However, EUV Direct printing comes with lots of challenges, especially at small pitches and tight tip-to-tip spacing. The combination between MP and EUV technologies will be the key factor in Moore’s law continuity in advanced nodes. Hence, this will enable Critical Dimension’s (CD) scaling down, and accordingly increases chips’ density, and provides more computational power chips. IMEC’s iN5 technology node, equivalent to industrial N3 technology node, uses SALELE Process to get critical features with tight tip-to-tip spacing in Back-End-Of-Line layers. SALELE process combines the two main MP approaches: (a) Self-Aligned MP and (b) Litho-Etch/Litho-Etch MP along with EUV technology. This combination creates a patterning methodology that overcomes EUV direct printing limitations and potentially creates a path towards scaling down. In this work we are introducing a manufacturing flow for the SALELE Process in details. Starting with layout decomposition, where the drawn layer is decomposed into 4 Masks: 2 Metal-like Masks, and 2 Block-like Masks. Then each of these masks is subjected to Optical Proximity Correction (OPC) process, and here we explain more about the OPC recipe development for each mask. Then we introduce a verification flow that performs two levels of verifications: (a) Litho verification, where the litho fidelity of each mask is quantified based on image quality measurements. (b) Final Manufactured shapes verification vs. expected output. This work has been carried out on an N3 candidate layout designed by IMEC.
Self-aligned quadruple patterning (SAQP) is not compatible with every design. It couldn’t pattern even number routing track, for examples 4 internal routing tracks with power rails side by side, due to the process footprint of conventional SAQP. On the other hand SAQP spacer merge technique is able to remove 1 internal metal line by merging 2 spacer into 1 spacer. It can offer additional track scaling and flexible design of track number, for example, 5.5 tracks together with 6.5 tracks to accomplish low and high performance device respectively. In this paper, SAQP spacer merge technique and self-aligned block (SAB) process are considered as one of potential patterning approaches for 1D style 28 nm metal pitch. SAQP spacer merge technique is indispensable for supporting 5.5T cell of 4 internal tracks with 28nm metal pitch. And 5.5T cell also requires the irregular metal color array for SAB and its biases which is litho-etch skew. SAB can be sized up double compared to conventional block process, it is biased over next metal line to takes advantage of material etch selectivity of SAQP structure inherently before metallization. To meets those requirement with automatic mask layout generation, we newly proposed forward decomposition algorithm and color-aware block resizing of SAB. The forward decomposition algorithm generates mandrel to spacer 1 to spacer 2 to mimics process order of SAQP spacer merge technique. And color-aware block resizing of SAB needs conditional bias depending on neighboring metal color. Additionally, edge placement error budget is analyzed with process variation band of source mask optimization (SMO) on top of overlay, line edge roughness (LER) and etch uniformity assumption. Simulation result seems to be fine to enable SAQP spacer merge and SAB integration. However, EUV stochastics reported that CD uniformity is not fit in Gaussian distribution. Considering beyond 3σ, restricted design rule may be needed. To see design availability, 3 representative standard library cells were verified in design rule restriction without area loss. This SAQP spacer merge decomposition algorithm is useful since it is possible to extend for Fin patterning application as well.
In the early 2000s, membranes both thin enough to transmit EUV light and strong enough to be free-standing at mask dimensions did not exist. The lithography community assumed that defect control for photomasks would be achieved, not with a pellicle, but with a clean scanner environment, thermophoretic protection and a removable pellicle.<sup>1</sup> In 2006, Intel published their research on an EUV pellicle.<sup>2</sup> Since then, an international development effort on EUV pellicle membranes has spanned a range of materials and fabrication approaches. Not only materials, but also the requirements of the EUV pellicle membrane have evolved over time. Imec’s pellicle work based on carbon nanotubes (CNTs) started in 2015, and is placed in relation to the rich history of EUV pellicles. CNTs are one-atom-thick carbon sheets rolled into tubes. The CNTs can be single- or multi-walled and can vary in diameter and in length. These engineered CNTs can be arranged in different configurations to form membranes of different densities. Thus, the CNT membrane’s properties can be fundamentally changed to meet the EUV pellicle targets for properties like transmittance. The historical trends in EUV pellicle membrane development are presented and the CNT membranes are described in that context.
In order to maintain the scaling trend in logic technology node progression, imec technology nodes started heavily utilizing design technology co-optimization (DTCO) on top of loosen pitch scaling trend to mitigate the burden from steep cost increase and yield challenge. Scaling boosters are adopted to enable DTCO process on top of patterning near its cliff to mitigate the cost increase. As the technology node further proceeds, DTCO also starts facing its cliff, and system technology co-optimization (STCO) is introduced to assist pitch and DTCO scaling to bridge 2-D IC technology to evolutionary technology options such as MRAM, 2.5-D heterogeneous integration, 3-D integration and 3-D IC. EUV is used to further assist pitch and DTCO scaling to maintain low cost with higher yield and faster turn-around-time (TAT). EUV single patterning, multiple patterning and high-NA EUV are considered on top of DTCO and STCO landscape to define imec technology nodes.
imec’s investigation on EUV single patterning insertion into industry 5nm-relevant logic metal layer is discussed. Achievement and challenge across imaging, OPC, mask data preparation and resulting wafer pattern fidelity are reported with a broad scope.
Best focus shift by mask 3D of isolated feature gets worse by the insertion of SRAF, which puts a negative impact on obtaining large overlap process window across features. imec’s effort across OPC including SMO and mask sizing is discussed with mask rule that affects mask writing. Resist stochastic induced defect is identified as a biggest challenge during the overall optimization, and options to overcome the challenge is investigated. For mask data preparation, dramatic increase in the data volume in EUV mask manufacturing is observed from iArF multiple patterning to EUV single patterning conversion, particularly by the insertion of SRAF. In addition, logic design consideration to make EUV single patterning more affordable compared to alternative patterning option is be discussed.
Conventional via patterning which relies on immersion ArF (iArF) lithography and self-aligned via (SAV) becomes
challenging in sub-7nm technology. EUV lithography (EUVL) is expected to achieve smaller feature
patterning thanks to its short wave length, but edge placement error (EPE) margin remains as another bottleneck
of pitch scaling; SAV can be aligned with metal on the top but not with the bottom of the via. Literary
study shows previous work on 2D self-aligned via (2D SAV) which can be aligned with the both metals, but it
cannot extend technology scaling beyond sub-5nm whose minimum metal pitch is expected as sub-20nm due to
essential limitation of EPE margin. We propose large marginal 2D SAV which has three times large EPE margin
than normal 2D SAV for extremely shrunk technology node (e.g. sub-5nm). Large marginal 2D SAV may allow
further feature size scaling, but it requires four EUV masks. Therefore, we present two count reduction methods
and corresponding mask decompositions and pattern re-targetings. Proposed re-targeted patterns are assessed
by source mask optimization (SMO) in terms of maximum EPE and process variation band (PVB) width.
EUV lithography (EUVL) is rising up as a solution of sub-10nm technology node via patterning. Due to better resolution of EUVL than it of immersion ArF (iArF) lithography, multiple iArF masks can be replaced by one EUV mask. However, for 24nm by 24nm metal grid, two diagonally neighboring vias yield either contour of two holes or peanut-shape contour. Because of the large variability of the via contours, the two vias are separably patterned with two different masks. We propose to insert bridge patterns (BPs) at the middle of the diagonally neighboring vias, so that single EUV exposure can draw peanut-shape contour consistently. In this study, we also assume 2D self-aligned via (2D SAV) which can align via holes in both vertical and horizontal direction for better edge placement error margin, so unique re-targeted via patterns that is called bridged via (BV) appears. We investigate impact of BV size and BP shapes on simulated contour using source mask optimization, and popular BVs are compared in terms of probability of failure which are calculated with Monte-Carlo simulation.
A protective membrane – a pellicle – must be used to prevent yield loss during EUV lithography exposure, just as it was for 193nm lithography. The pellicle must be thin enough to transmit EUV light, yet strong enough to withstand the scanner environment. Membrane solutions for ~ 80W exposure exist. Our focus is developing a membrane solution for 250W exposure power. The main pellicle challenge here is still the identification of a membrane material that has very high transmission at EUV wavelengths. Additionally, absorption during lithographic exposure results in high thermal and mechanical load for the pellicle, which can cause yield problems. The current candidates for pellicle membranes such as poly-silicon and silicon nitride cannot withstand 250W power conditions, therefore alternative materials will be required for the future HVM pellicle. <p> </p>At imec, a variety of novel membrane material options are investigated for the HVM pellicle application. One promising approach is based on carbon nanotubes (CNT). In this paper we outline different CNT based process options, and report results on their optical, thermal, and mechanical performance. In addition, we will report on their uniformity and robustness towards scanner application. Finally, the family of CNT-based membrane options will be compared to promising candidates fabricated using conventional film approaches that do not have a CNT layer.
EUV lithography insertion is anticipated at the 7 nm node and below; however, defects added to the mask during use is a
lingering concern. Defectivity in the scanner is non-zero and an EUV pellicle membrane to protect the mask for high
volume manufacturing power levels does not yet exist. The EUV photons are strongly absorbed by all materials. Sibased
membranes leverage the low absorption coefficient k value (k = 0.0018 at 13.5 nm) for reasonable transmission,
but poly Si becomes fragile and wrinkles during the high temperatures associated with exposure. An alternate approach
to high transmission is deploying very thin or porous layers so that there are fewer atoms to absorb light. For example,
carbon nanomaterials have a reasonably low k value (k = 0.0069), but are strong enough to be fabricated in very thin
layers. Graphene, graphite, carbon-nanosheets and carbon nanotubes are all candidate carbon nanomaterials for this
application, but we focus here on carbon nanotubes (CNTs). Our first measurements on CNT films of ~60 nm thick were
found to have 96.5% transmission at 13.5 nm. Adding CNT layers also enhanced the strength of a thin SiN membrane
significantly. In this paper, critical pellicle metrics will be evaluated in more detail: EUV transmission, bulge test for
mechanical strength, emissivity measurements for heat management, and exposure testing in a hydrogen environment.
EUV mask protection against defects during use remains a challenge for EUV lithography. A stand-off protective membrane – a pellicle – is targeted to prevent yield losses in high volume manufacturing during handling and exposure, just as it is for 193nm lithography. The pellicle is thin enough to transmit EUV exposure light, yet strong enough to remain intact and hold any particles out of focus during exposure. The development of pellicles for EUV is much more challenging than for 193nm lithography for multiple reasons including: high absorption of most materials at EUV wavelength, pump-down sequences in the EUV vacuum system, and exposure to high intensity EUV light. To solve the problems of transmission and film durability, various options have been explored. In most cases a thin core film is considered, since the deposition process for this is well established and because it is the simplest option. The transmission specification typically dictates that membranes are very thin (~50nm or less), which makes both fabrication and film mechanical integrity difficult. As an alternative, low density films (e.g. including porosity) will allow thicker membranes for a given transmission specification, which is likely to improve film durability. The risk is that the porosity could influence the imaging. At imec, two cases of pellicle concepts based on reducing density have been assessed : (1) 3D-patterned SiN by directed self-assembly (DSA), and (2) carbon nanomaterials such as carbon nanotubes (CNT) and carbon nanosheets (CNS). The first case is based on SiN membranes that are 3D-patterned by Directed Self Assembly (DSA). The materials are tested relative to the primary specifications: EUV transmission and film durability. A risk assessment of printing performance is provided based on simulations of scattered energy. General conclusions on the efficacy of various approaches will provided.
EUV mask protection during handling and exposure remains a challenge for high volume manufacturing using EUV scanners. A thin, transparent membrane can be mounted above the mask pattern so that any particle that falls onto the front of the mask is held out of focus and does not image. The fluoropolymer membranes that are compatible with 193nm lithography absorb too strongly at the 13.5nm EUV exposure wavelength to be considered. Initially, the industry planned to expose EUV masks without any pellicle; however, the time and cost of fabricating and qualifying an EUV mask is simply too high to risk decimating wafer yield each time a particle falls onto the mask pattern. Despite the challenges of identifying a membrane for EUV, the industry has returned to the pellicle concept for protection. EUVL pellicles have been in development for more than a decade and reasonable options exist. Meeting all pellicle requirements is difficult, so this type of risk-mitigation effort is needed to ensure that there is a viable high-volume manufacturing option. This paper first reviews the desired membrane properties for EUVL pellicles. Next, candidate materials are introduced based on reported properties and compatibility with fabrication. Finally a set of candidate membranes are fabricated. These membranes are screened using a simplified set of tests to assess their suitability as an EUV pellicle. EUV transmission, film stress, and film durability data are included. The results are presented along with general guidelines for pellicle membrane properties for EUV manufacturing.
The resist underlayer (UL) has been shown to beneficially impact the exposure latitude in photolithography techniques.
As a result, the development of the resist UL is in progress for extreme ultraviolet lithography (EUVL) as well. Since the
aspect ratio of patterns increases as the feature size decreases, a high-performance EUV UL is expected to be in high
In this study, we evaluated the optical properties of the EUV UL by using the lithography simulation tool PROLITH X5
(KLA-Tencor). We quantified the imaging properties of a 14 nm half-pitch (HP) line and space (L/S) pattern by varying
the refractive index, extinction coefficient and thickness of the UL under 0.5 numerical aperture (NA) conditions with a
conventional binary intensity mask.
These simulations reveal that the number of photons absorbed in the photoresist increases as the refractive index of the
UL decreases; this results from the increase in reflectivity from the UL/photoresist interface. Therefore, the line critical
dimension (CD) mean value decreases and stochastic imaging properties improve in the observation plane. As the
refractive index of the UL is reduced, however, the light intensity in resist and the distribution of photons is distorted by
the standing wave effect, resulting in roughness and non-uniformity in the pattern sidewall. Therefore, the refractive
index of the UL should be similar to that of the photoresist in order to get the optimized performance.
The half-tone phase shift mask (PSM) has been suggested for better imaging performances like image contrast,
NILS and H-V bias compared to the binary mask (BIM) in EUV lithography. In this paper, we measured
imaging performance of a fabricated half-tone attenuated PSM with Coherent Scattering Microscopy (CSM) and
the results were compared with simulation data obtained by EM-suite tool. We prepared a half-tone attenuated
PSM which has 12.7% reflectivity and 180° phase shift with absorber stack of 16.5mn-thick TaN absorber and
24nm-thick Mo phase shifter. With CSM, an actinic inspection tool, we measured the imaging properties of
PSM. The diffraction efficiencies of BIM were measured as 31%, 36%, and 44% for 88 nm, 100 nm, and 128
nm mask CD, respectively, while those of PSM were measured as 45%, 62%, and 81%. Also the aerial image at
wafer level obtained by CSM with high volume manufacturing tool’s (HVM) illumination condition (NA=0.33,
σ=0.9) showed higher image contrast and NILS with phase shift effect. And the measured data were consistent
with the simulation data.
In EUV Lithography, mask shadowing effect and photon shot noise effect are the main sources of patterning limit,
critical dimension (CD) non-uniformity and low imaging properties. In this paper, the patterning performance of a 6%
attenuated phase shift mask (PSM) is valuated, and the results show that this can be used for half-pitch (hp) down to 14 nm with 0.33NA due to the improved stochastic patterning properties. The proposed PSM consists of 26.5 nm of TaN as an absorber layer and 14 nm of Mo as a phase shifter on 2.5 nm thick Ru capped Mo/Si multilayers. This structure has ~6% reflectivity at the absorber stack and 180° phase shift. The improved stochastic resist patterning properties of PSM were compared with those of conventional binary intensity mask (BIM) with a 70 nm-thick TaN absorber for the 14 ~ 22 nm line and space (L/S) 1:1 dense pattern with 0.33NA off-axis illumination conditions with a EUV generic resist model.