How positive tone development works — and where it breaks down
In positive tone development (PTD), EUV-exposed regions of a chemically amplified resist (CAR) become soluble and are removed by an aqueous alkaline developer — typically tetramethylammonium hydroxide (TMAH) — leaving behind the unexposed areas as the physical pattern. The photochemical chain begins with EUV photons generating photoelectrons that decompose photoacid generators (PAGs), liberating acid that catalytically deprotects polymer protecting groups during the post-exposure bake (PEB). The deprotected, polar regions are then dissolved by the aqueous developer. This process has been modeled computationally by Osaka Prefecture University (2019), with development represented as the removal of polymers exceeding a deprotection conversion ratio threshold.
PTD has been the workhorse of optical lithography for decades, and as noted by IMEC in their 2022 CDSEM metrology study, CAR remains the standard photoresist platform through successive EUV technology nodes. However, PTD using aqueous alkaline developers faces a critical physical limitation at sub-20 nm half-pitch: polymer swelling. The water-based developer causes partial swelling of resist features during development, introducing dimensional instability and roughness at feature sidewalls. FUJIFILM Corporation’s 2015 comparative study of PTD and NTD approaches confirmed that the non-swelling character of organic solvent developers used in NTD confers measurable advantages in resolution and LWR in the 30–45 nm half-pitch range.
Sensitivity in PTD is governed by the quantum yield of acid generation per absorbed EUV photon. Low-energy secondary electrons generated during EUV photon absorption are critical drivers of PAG decomposition, and their diffusion length carries direct implications for both resolution and line edge roughness (LER), as documented by the University at Albany (2014). The fundamental challenge is that increasing sensitivity — via higher acid yields or longer diffusion lengths — degrades resolution and LWR. This is the classic resolution-LWR-sensitivity (RLS) trilemma. SEMATECH’s 2014 review confirmed that progress in CAR had decelerated as stochastic limits in photon counts and material parameters were approached at nodes below 14 nm half-pitch.
The RLS trilemma is the fundamental trade-off in EUV resist design between Resolution, Line-width Roughness (LWR), and Sensitivity. Improving any one parameter tends to degrade the other two. PTD-based CAR processes face this constraint most acutely because aqueous development adds an extrinsic swelling contribution to LWR on top of the intrinsic photochemical noise floor.
In positive tone development (PTD) for EUV lithography, exposed resist regions are dissolved by aqueous alkaline TMAH developer, but polymer swelling during aqueous development directly increases line-width roughness (LWR) and limits resolution at sub-20 nm half-pitch, as documented by FUJIFILM Corporation (2015) and SEMATECH (2014).
Negative tone development: mechanism, chemistry, and LWR advantages
Negative tone development (NTD) inverts the solubility logic of PTD: exposed regions are rendered insoluble through crosslinking, while unexposed regions are removed by an organic solvent developer. The most extensively studied organic developer for EUV-NTD is n-butyl acetate (nBA). EIDEC’s 2015 comprehensive study established that EUV negative-tone imaging uses organic solvent-based developers to provide low swelling and smooth-dissolving behavior, which directly translates into improved LWR compared to PTD while maintaining sensitivity — with a high-sensitivity NTD formulation resolving 22 nm half-pitch on an NXE:3100 scanner at single-digit mJ/cm² photo speed.
“NTD using n-butyl acetate achieves better LWR than PTD while maintaining sensitivity, with single-digit mJ/cm² photo speed demonstrated at 22 nm half-pitch on an NXE:3100 scanner — a landmark result for the NTD approach.” — EIDEC, 2015
The mechanism in negative-type CARs under EUV exposure has been computationally modeled by Osaka Prefecture University (2020). In this framework, EUV exposure activates PAGs according to deposited energy distributions computed by Monte Carlo photoelectron scattering simulation. Acid diffusion then drives polymer crosslinking reactions during PEB, and development removes polymers with a polymerization degree below the crosslink density threshold. This crosslink-based development criterion is fundamentally different from the deprotection-ratio threshold in positive CARs, conferring greater chemical contrast at the pattern edge.
A critical constraint of NTD is its sensitivity to the developer solubility parameter. FUJIFILM’s 2015 study showed that solvents with lower solubility parameters reduce bridging defects at sub-20 nm half-pitch, but an excessively low solubility parameter causes film remaining due to an extremely slow maximum dissolution rate (Rmax). This reveals a process window constraint for NTD without a direct equivalent in PTD: the organic developer must dissolve unexposed regions without swelling or prematurely attacking the exposed crosslinked features. FUJIFILM’s 2014 study further correlated surface roughness of non-patterned resist films at half-exposed areas with process outcomes, establishing nBA as the preferred developer for dissolution uniformity.
Negative tone development (NTD) for EUV lithography uses n-butyl acetate (nBA) as the primary organic solvent developer. Unlike aqueous TMAH used in PTD, nBA exhibits low swelling and smooth-dissolving behavior, which EIDEC’s 2015 study identified as the primary mechanistic reason NTD achieves superior line-width roughness (LWR) compared to PTD at half-pitches above 20 nm.
An alternative NTD approach based on non-chemically amplified resists (n-CARs) has also been explored. Research from Universidade Federal do Rio Grande do Sul (2014) reported that MAPDST homopolymer and MAPDST-MMA copolymer n-CAR negative resists demonstrate lithographic performance at sub-20 nm, relying on radiation-sensitive sulfonium functionality rather than acid-catalyzed amplification chains. Such materials avoid the acid diffusion blur inherent to CARs but typically suffer from lower sensitivity — a direct expression of the RLS trilemma in the n-CAR context.
Explore the full patent landscape for EUV resist chemistry and NTD process innovations in PatSnap Eureka.
Explore EUV Resist Patents in PatSnap Eureka →PTD vs. NTD head-to-head: resolution, defects, and process complexity
PTD and NTD differ across five key performance dimensions at advanced EUV nodes: developer chemistry, resolution behavior, sensitivity and RLS trade-off, process complexity, and defect modes. Understanding each dimension is essential for process engineers selecting a development approach for sub-22 nm patterning.
Developer chemistry and LWR
PTD uses aqueous alkaline developers (TMAH), which cause polymer swelling in the resist and directly increase LWR. NTD uses organic solvent developers (primarily nBA), which exhibit low swelling and smooth-dissolving behavior. EIDEC’s 2015 study identified this as the primary mechanistic reason NTD achieves superior LWR, particularly at half-pitches above 20 nm. The NCSR Demokritos 2020 review of high-sensitivity EUV resists confirmed that NTD offers a partial route around the RLS trilemma by improving LWR without necessarily sacrificing sensitivity, since the non-swelling organic solvent development reduces a major LWR contribution that is extrinsic to the resist photochemistry.
Defect modes: pattern collapse vs. bridging
PTD is susceptible to pattern collapse — lines falling over due to surface tension forces during aqueous rinse — especially at high aspect ratios. NTD is susceptible to bridging, where unexposed resist fails to fully dissolve between features, and to residue formation at the tightest pitches where developer penetration is limited. FUJIFILM’s 2015 analysis showed that bridge formation in NTD is controlled by the polarity parameter of the organic developer, and that an excessively low solubility parameter causes film remaining due to extremely slow maximum dissolution rate. PTD is limited at the tightest pitches by aqueous swelling-induced pattern deformation such as collapse and pinching, as shown in FUJIFILM’s 2014 study on 14 nm half-pitch resist design.
Both PTD and NTD are governed by EUV photon shot noise and secondary electron stochastic effects. Osaka University’s 2014 study confirmed that optical contrast degradation more strongly affects stochastic defect generation than LER — a conclusion that applies equally to both tone processes. NTD’s reduced extrinsic LWR from swelling may allow dose to be directed more efficiently toward noise management, but it does not eliminate shot noise as a fundamental constraint.
Process complexity
PTD maps directly onto decades of aqueous development infrastructure, making it simpler from a process integration standpoint. NTD requires organic solvent handling, different track configurations, and careful tuning of developer solubility parameters. According to IMEC‘s 2019 photoresist challenge assessment for 0.33NA EUV, the leading challenge across both tone processes is achieving low defect density at doses compatible with throughput. IBM Research’s 2019 study further demonstrated that interfacial effects between resist and hardmask play a dominant role in material stochastics at sub-32 nm pitch, with distinct implications for each tone: in NTD the exposed crosslinked material remains and mechanically couples to the hardmask differently than in PTD where the exposed material is dissolved away.
In EUV lithography, positive tone development (PTD) is susceptible to pattern collapse during aqueous rinse due to surface tension forces, while negative tone development (NTD) is susceptible to bridging — where unexposed resist fails to fully dissolve between features at tight pitches. Developer solubility parameter tuning is essential for NTD to avoid both bridging and film remaining from an excessively slow maximum dissolution rate, as documented by FUJIFILM Corporation (2015).
Production implementations: TSMC, Tokyo Electron, and dual-tone approaches
TSMC has been a leading patent assignee in NTD process implementation for production-grade EUV patterning. Their 2017 and 2018 patents disclose a method wherein the EUV resist is intentionally exposed at a dose below the target threshold, producing an opening with a critical dimension (CD) larger than desired. Unexposed portions are then removed by NTD, and an interfacial layer is conformally deposited on sidewalls to shrink the opening to the target CD. This combined NTD-plus-shrink approach decouples dose from final CD, enabling dose reduction — directly addressing EUV source power constraints — while maintaining dimensional control. This advantage is not achievable with PTD in the same manner, since PTD removes exposed material and the CD relationship to dose is non-invertible without additional process steps.
TSMC’s 2017 and 2018 EUV NTD patents disclose a process where the resist is exposed below the target dose threshold, unexposed portions are removed by negative tone development, and an interfacial layer is conformally deposited on sidewalls to shrink the critical dimension to target — decoupling dose from final CD and enabling dose reduction while maintaining dimensional control.
Tokyo Electron Limited holds multiple active patents across PTD and NTD adjacent technologies, including a “reverse patterning” technique that incorporates overcoating, etch-back or planarization, and pattern removal steps beyond conventional NTD. This approach specifically targets microbridge defect elimination and LWR reduction, combining the NTD process with a line-smoothing sequence to achieve a target roughness level that is difficult to reach with NTD or PTD alone.
The 2010 Dual Tone Development patent by Mark Somervell (WO) describes a unified framework using resists with polymer backbones bearing multiple protecting groups, enabling both PTD and NTD within the same material system. By controlling developer chemistry — aqueous alkaline for PTD, organic solvent for NTD — the same resist can produce complementary tone patterns. This dual-tone capability is highly relevant for advanced node design flexibility, particularly for contact hole versus pillar printing from the same mask, as noted in the patent’s claims. The PatSnap Eureka platform enables engineers to trace these patent families and identify continuation filings across jurisdictions.
Track TSMC, Tokyo Electron, and FUJIFILM EUV resist patent families — including NTD continuations — in PatSnap Eureka.
Analyse EUV Patent Families in PatSnap Eureka →Beyond conventional CARs: molecular resists and the PEB-free frontier
The most significant recent development in NTD resist design is the elimination of the post-exposure bake (PEB) step entirely. The Multi Trigger Resist (MTR), developed at the University of Birmingham, is a negative-tone crosslinking molecular resist that does not require PEB. This eliminates thermally driven acid diffusion blur — a source of LWR degradation in both PTD and NTD CAR systems. The MTR2 formulation patterned 16 nm half-pitch lines on an ASML NXE:3300B with LER of 3.7 nm at 38 mJ/cm², as reported in the University of Birmingham’s 2018 publication. A 2017 companion study from the same group reported sensitivity enhancement work for the xMT multi-trigger resist, further developing the platform.
“The MTR2 formulation patterned 16 nm half-pitch lines on an ASML NXE:3300B with LER of 3.7 nm at 38 mJ/cm² — without any post-exposure bake, eliminating thermally-driven acid diffusion blur as a LWR source.” — University of Birmingham, 2018
The omission of PEB in MTR is a fundamental process simplification relative to conventional CAR-based NTD. In standard NTD CARs, the PEB drives acid diffusion that catalyses crosslinking, but thermal diffusion also blurs the latent chemical image, contributing to LWR. By using a multi-trigger crosslinking mechanism that responds directly to radiation without requiring thermally activated acid catalysis, the MTR approach decouples patterning fidelity from PEB temperature uniformity — a significant advantage for wafer-level process control. According to NIST metrology standards applied to EUV patterning, sub-4 nm LER at 16 nm half-pitch represents a meaningful advance toward the targets required for high-NA EUV production.
IMEC’s 2017 assessment of photo material readiness at the eve of EUV high-volume manufacturing (HVM) placed both PTD and NTD within the broader context of resist platform selection for logic and memory. The study confirmed that no single resist platform — PTD CAR, NTD CAR, or molecular NTD — satisfies all requirements simultaneously, and that the optimal choice depends on the specific patterning application: line-space arrays, contact holes, or via patterns. For line-space patterns where LWR is the dominant specification, NTD holds a clear advantage. For contact hole printing where collapse risk is a primary concern, NTD’s resistance to aqueous surface tension forces provides an additional benefit. The PatSnap Analytics platform enables R&D teams to map these technology readiness trajectories across assignee portfolios and filing dates.
Looking forward, the convergence of NTD chemistry with reverse patterning sequences (Tokyo Electron), interfacial layer CD control (TSMC), and PEB-free molecular crosslinking (University of Birmingham) suggests that production-grade EUV patterning at sub-14 nm half-pitch will require hybrid process architectures rather than a single development paradigm. According to SEMATECH‘s 2014 analysis, the deceleration of CAR progress as stochastic limits are approached makes these architectural innovations — rather than incremental resist chemistry optimization — the critical path for continued scaling.