Here we examine a new methodology for spatially resolved molecular analysis to address an important area of need related to the performance of EUV resists, namely detection of molecular components at a scale below the current 20 nm critical dimension. Analysis consists of stochastically sampling the surface with a suite of nano projectiles each of which results in the emission of multiple secondary ions, SIs, from a volume 10-15 nm in diameter on the surface. The SI detector is a multi-anode detector allowing for the radial and translational energies of each SI to be examined. We tested this new methodology on model line patterned EUV resist materials and found the when bombarding the surface with the primary ion path perpendicular to the line pattern, impacts on top of the intact resist resulted in the emission of SI with high translational and axial energies in the forward direction. In contrast, impacts which occurred on the resist sidewalls results in emission in the backward direction. Thus, by examining the energies of emitted SIs molecular analysis can be undertaken on each part of the pattern (side wall, intact resist, substrate) with high resolution.
Resists are needed to advance extreme ultraviolet (EUV) lithography. In EUV resists, due to the high energy of the incident photons, most of the chemistry arises from the emitted primary and secondary electrons and not the EUV photons themselves. Because the electrons are playing a leading role in EUV patterning, initiating chemical transformations, it is important to characterize their generation, transport, and energy distribution. In this work, we present several experimental techniques to probe model polymer materials to investigate the impact of specific chemical groups on critical resist properties: EUV absorption, electron emission, electron attenuation length (EAL), and energy distribution of emitted electrons. Total electron yield provides information on the conversion of absorbed EUV photons to electrons, and photoelectron spectroscopy provides information on energy distribution of generated electrons. The EAL reveals the distance that the electrons can travel in a resist film, which is related to the electron blur. Correlations between the obtained experimental values are discussed. We explore how different elements or functional groups change the yield, EAL, and energy distribution of emitted electrons, aiming to understand how to control the electron cascade.
Here, we present a methodology for identifying and characterizing nanoscale sites in extreme ultraviolet (EUV) photoresists that deviate from the mean composition by 3σ. The methodology is based on nano-projectile secondary ion mass spectrometry (SIMS) operating in the event-by-event bombardment detection mode. Nanoscale analysis is achieved by probing the surface stochastically with a suite of individual nanoprojectile impacts in which each nano-projectile samples a volume that is 10 to 15 nm in diameter and up to 10 nm in depth. For each impact, the coemitted secondary ions are collected and mass-analyzed, allowing for the analysis of colocalized moieties. We applied this technique to study the fundamental processes occurring in partially developed positive-tone EUV resists, which simulate a critical problem in EUV resists, incomplete resist removal, and production of line edge features. Such features erode device yield and have been the focus of many previous studies. Using NP-SIMS, we examined the changing molecular composition in the partially developed resist and isolated measurements with a probability below 0.3%. Grouping measurements based on the number and type of detected molecular species allowed for the identification of rare sites on the surface that deviate from the mean composition. The mass spectrometry measurements showed that both the photoacid generator (PAG) cation and anion displayed decreased homogeneity on average with increasing exposure dose. The effect was more pronounced in the sites with probabilities below 0.3%, where the measured intensity of the PAG-related ions in these sites was more than twofold larger than the mean. Thus, we attribute these nanoscale sites to aggregations of PAG within the top 10 nm of the remaining film. These results suggest that identifying and characterizing the molecular composition of rare sites may be important in defect production and film stochastics.
In this study we examined a series of model EUV resists with varying concentration of photoacid generator, PAG, or photodecomposable quencher, PDQ and their effects on resist uniformity using Nano-Projectile Secondary Ion Mass Spectrometry, NP-SIMS. Nanoscale analysis with NP-SIMS is achieved due three innovative features of the technique (1) the mode of data acquisition (2) method of data analysis (3) the nature of the impacting projectile. The results showed that ionic interactions between PAG and PDQ are modified due to the relative/absolute loading of the two components. These results demonstrate the NP-SIMS is a useful tool for assessing the effect of additive loadings on EUV resist uniformity at the nanoscale.
Characterizing chemical changes in photoresists during fabrication processes is critical to understanding how nanometric defects contribute to film stochastics. We used nanoprojectile secondary ion mass spectrometry (NP-SIMS) to evaluate the nanoscale homogeneity of components in positive-tone extreme ultraviolet resists. NP-SIMS was operated in the event-by-event bombardment/detection mode, where a suite of individual gold nanoprojectiles separated in time and space stochastically bombard the surface. Each impact ejects secondary ions from a volume 10 to 15 nm in diameter and up to 10 nm in depth allowing for analysis of colocalized moieties with high spatial resolution. Individual partially exposed extreme ultraviolet resists were analyzed after light exposure, postexposure bake, and development. Results showed an expected increase in protonated quencher versus exposure dose, while after development, we observed increased abundance in the remaining film. The latter, we attribute to poor solubility in the developing solvent. Examining the photoacid generator (PAG), we found decreased PAG cation abundance versus exposure dose in the exposed films, likely due to photodecomposition of the PAG cation. Moreover, after the development, we observed decreased homogeneity of PAG ions, which we attribute to preferential extraction caused by ion-exchange interactions with the developer. We found that the insoluble moieties persisting on the surface after the development were relatively rich in the protecting group, likely due to uneven deprotection of the polymer. Overall, NP-SIMS allows to characterize the resist at the nanoscale and identify conditions that lead to defect formation.
Here we present a methodology for identifying and characterizing nanoscale sites in EUV photoresists which deviate from the mean composition by 3σ. The methodology is based on Nano-Projectile Secondary Ion Mass Spectrometry (SIMS) operating in the in the event-by-event bombardment detection mode. Nanoscale analysis is achieved by probing the surface stochastically with a suite of individual nano-projectile impacts where each nano-projectile samples a volume 10-15 nm in diameter and up to 10 nm in depth. For each impact the coemitted secondary ions are collected, and mass analyzed, allowing for the analysis of co-localized moieties. We applied this method to study the changing film composition in an EUV resist and isolated measurements with a probability below 0.3%. By examining these measurements, we can identify rare sites on the surface that may correspond to molecular aggregations in the surface. In a developed film, the mass spectrometry measurements showed that the photoacid generator, PAG, cation displayed decreased homogeneity on average with increasing exposure dose. The effect was more pronounced in the sites with probabilities below 0.3%, where the measured intensity of the PAG cation in these sites was over 2-fold larger than the mean. Thus, we attribute these nanoscale sites to aggregations of PAG within the top 10 nm of the film. These rare sites may be important in defect production and film stochastics.
Characterizing chemical changes in photoresists during fabrication processes is critical to understanding how nanometric defects contribute to film stochastics. We used Nano-Projectile Secondary Ion Mass Spectrometry (NP-SIMS) to evaluate the nanoscale homogeneity of components in positive-tone extreme ultraviolet resists. NP-SIMS was operated in the event-by-event bombardment/detection mode, where a suite of individual gold nanoprojectiles separated in time and space stochastically bombard the surface. Each impact ejects secondary ions from a volume 10-15 nm in diameter and up to 10 nm in depth allowing for analysis of colocalized moieties with high spatial resolution. Individual partially exposed EUV resists were analyzed after light exposure, postexposure bake (PEB), and development. Results showed an expected increase in protonated quencher versus exposure dose, while after development we observed increased abundance in the remaining film. The latter we attribute to poor solubility in the developing solvent. Examining the photoacid generator, PAG, we found decreased PAG cation abundance versus exposure dose in the exposed films, likely due to photodecomposition of the PAG cation. Moreover, after development we observed decreased homogeneity of PAG ions, which we attribute to preferential extraction caused by ion-exchange interactions with the developer. We found that the insoluble moieties persisting on the surface after development were relatively rich in the protecting group, likely due to uneven deprotection of the polymer. Overall, NP-SIMS allows to characterize the resist at the nanoscale and identify conditions that lead to defect formation.
New resists are needed to advance EUV lithography. Tailored design of efficient photoresist is impossible without fundamental understanding of EUV induced chemistry. The absorption of an EUV photon by a thin film resist leads to emission of primary and secondary electrons. The electrons may travel up to tens of nanometers before losing their kinetic energy via collisions which initiate chemical reactions. The “blur” of an aerial image is directly related to the distance that electrons are able to travel and initiate chemistry in the resist. Thus, identifying how to measure and influence the absorption of EUV photons, emission of electrons, and distance traveled by the secondary electrons is extremely beneficial to the resist community.
In this work, we present several experimental techniques to probe model polymer materials to investigate the impact of specific chemical groups on three critical resist properties: EUV absorption, electron emission, and the electron attenuation length (EAL). EUV absorption dictates the efficiency of the film to absorb photons. Total electron yield (TEY) provides information on the conversion of absorbed EUV photons to electrons, whereas photoelectron spectroscopy (PES) provides information on energies and abundance of generated electrons. The EAL corresponds to the thickness of a material required to reduce the number of emitted electrons to 1/e of the initial value. The EAL reveals the distance the electrons can travel in a resist film, which is directly related to the electron blur. Correlations between the obtained experimental values is discussed.
Despite years of research and development, the fundamental processes of photoionization, secondary electron generation, recombination, diffusion, and resist switching are poorly understood at the atomic level for EUVL. Multiscale modeling of these physical and chemical processes can provide answers to questions that are difficult or impossible to answer with experiment alone. A modeling pipeline that includes Monte Carlo modeling of photon- and electron-matter interactions, along with density functional theory calculations of chemical switching will be introduced in this proceeding. The Hf4O2(OMc)12 nanocluster resist will be presented as a case study. Photon and secondary electron yields, electron energy and spatial distributions, and a quantum chemical pathway for negative tone switching will be presented. Fundamental learning from studies like this can be used to improve resist design including improving contrast of these materials.
Extreme ultraviolet lithography (EUVL) technology continues to progress and remains a viable candidate for next generation lithography1, which drives the need for EUV resists capable of high resolution with high sensitivity and low LWR. While chemically amplified resists (CARs) have demonstrated the ability to pattern 12nm half-pitch features2, pattern collapse continues to limit their ultimate resolution. We have taken multiple approaches to extend resist capabilities past these limits. Recent results in pattern collapse mitigation using a resist encapsulation and etch back strategy will be discussed. We continue to investigate EUV patterning of semi-inorganic resists to simultaneously increase EUV photon absorption and extend mechanical strength beyond CAR capabilities. The limitations of metal oxide-based nanoparticle photoresists have been investigated, and have provided key insights to further understanding the mechanism of this class of materials.
Recently, both PSI1 and ASML2 illustrated champion EUVL resolution using slow, non-chemically amplified inorganic resists. However, the requirements for EUVL manufacturing require simultaneous delivery of high resolution, good
sensitivity, and low line edge/width roughness (LER/LWR) on commercial grade hardware. As a result, we believe that
new classes of materials should be explored and understood. This paper focuses on our efforts to assess metal oxide based
nanoparticles as novel EUV resists3. Various spectroscopic techniques were used to probe the patterning
mechanism of these materials. EUV exposure data is presented to investigate the feasibility of employing inorganic
materials as viable EUV resists.
One of the key challenges to high resolution resist patterning is pattern collapse. Using a new scanning probe microscopy (SPM), Peak ForceTM tapping, we map nano-mechanical properties-- modulus, adhesion, and dissipation-- of the exposed/developed resist structures with sub-10 nm resolution. Properties are compared across a carbon based negative resist with and without cross-linking. The SPM technique reveals that cross-linking significantly enhances the mechanical properties to give a champion resolution of sub 20 nm half-pitch in a chemically amplified negative resist system. Beyond mechanical properties, surface morphology and redistribution kinetics were examined using complementary techniques and reveal additional benefits with cross-linking.
Here, we report the highest recorded resolution for a negative-tone, carbon-based, chemically amplified (CA) resist of 20 nm half-pitch (HP) using both E-beam and EUV exposure systems. The new chemistry incorporates variable amounts of oxetane (0, 5, 10 and 20%) cross-linker into a base of Noria-MAd (methyl-admantane) molecular resist. Cross-linkable resists showed simultaneous improvements in surface energy, structural integrity, and swelling to ensure collapse free 20nm HP patterns and line-edge roughness (LER) down to 2.3 nm. EUV exposed Noria-Ox (5%) cross-linked resist patterns demonstrated 5 times improvement in Z-factor (for 24 nm HP) over Noria-MAd alone.
Modern high-resolution lithography, which employs a chemically amplified resist (CAR) at either 193 or 13.5 nm wavelength, is often limited by pattern collapse. While the general concepts of how CAR platforms work are widely understood, the influence of composition on pattern collapse has been studied to a lesser extent. In addition, the subject is often further complicated by non-disclosure of the resist chemistry used in the lithographic evaluation. Open-source photoresist platforms can be beneficial for fundamental studies on how individual components influence pattern collapse. Such platforms should mimic a typical CAR, containing-apart from the polymer-additional components such as photo acid generators (PAGs) and base quenchers. Here, 193 nm and extreme ultraviolet lithography open-source platforms are presented wherein the chemistry, composition, and concentration are all disclosed. With the aim of fundamentally understand how resist composition and behavior influences pattern collapse, the molecular weight of the polymer backbone and the concentration of both PAG and base quencher were varied. These sets of resists were exposed using high-end optical lithography scanners. The results are presented such that the probability of pattern collapse is derived as a function of the exposure wavelength, chemistry, and component concentrations.
Modern high-resolution lithography, which employs a chemically amplified resist (CAR) at either 193 or 13.5 nm
wavelength, is often limited by pattern collapse. While the general concepts of how CAR platforms work are widely
understood, the influence of composition on pattern collapse has been studied to a lesser extent. In addition, the subject
is often further complicated by non-disclosure of the resist chemistry used in the lithographic evaluation. Open-source
photoresist platforms can be beneficial for fundamental studies on how individual components influence pattern collapse.
Such platforms should mimic a typical CAR, containing - apart from the polymer - additional components such as photo
acid generators (PAGs) and base quenchers. In this paper, 193 nm and EUVL open-source platforms are presented
wherein the chemistry, composition, and concentration are all disclosed. With the aim to fundamentally understand how
resist composition and behavior influences pattern collapse, the molecular weight of the polymer backbone and the
concentration of both PAG and base quencher were varied. These sets of resists were exposed using both high-end
optical lithography scanners. The results are presented such that the probability of pattern collapse is derived as a
function of the exposure wavelength, chemistry, and component concentrations.
Current work in lithographic patterning has been carried out using 193 nm excitation sources, limiting the pitch division
to approximately λ/2 and, thus, the advancement of Moore's law. Recently, double patterning has emerged as a potential
extension of 193 nm techniques as two lines can be patterned in one exposure. In this contribution, the double patterning
features of single component carbamate photoacid/photobase generators (PAG/PBG) are examined. At lower exposure
doses, sulfonic acid is generated, while at higher doses, a photochemical rearrangement is initiated to activate the PBG.
Optimally, at intermediate doses, photoacid and photobase components can exist concurrently resulting in the desired
dual tone lithographic features. The energy required to initiate dual tone behavior can be tailored through co-added
amine quenchers and carbamate concentration. Using ellipsometry, the energy required for the resists to have the first
sign of photoacid generation (film dissolution), E0, and at the energy required for photobase activation (En) were
determined, as this value dictates the ability to achieve the desired pitch division.
Pitch division lithography (PDL) with a photobase generator (PBG) allows printing of grating images with twice
the pitch of a mask. The proof-of-concept has been published in the previous paper and demonstrated by
others. Forty five nm half-pitch (HP) patterns were produced using a 90nm HP mask, but the image had line
edge roughness (LER) that does not meet requirements. Efforts have been made to understand and improve the
LER in this process. Challenges were summarized toward low LER and good performing pitch division.
Simulations and analysis showed the necessity for an optical image that is uniform in the z direction in order for
pitch division to be successful. Two-stage PBGs were designed for enhancement of resist chemical contrast. New
pitch division resists with polymer-bound PAGs and PBGs, and various PBGs were tested. This paper focuses on
analysis of the LER problems and efforts to improve patterning performance in pitch division lithography.
The semiconductor industry is pursuing several process options that provide pathways to printing images smaller
than the theoretical resolution limit of 193 nm projection scanners. These processes include double patterning, side
wall deposition and pitch division. Pitch doubling lithography (PDL), the achievement of pitch division by addition
of a photobase generator (PBG) to typical 193 nm resist formulations was recently presented.1 Controlling the net
acid concentration as a function of dose by incorporating both a photoacid generator (PAG) and a PBG in the resist
formulation imparts a resist dissolution rate response modulation at twice the frequency of the aerial image.
Simulation and patterning of 45 nm half pitch L/S patterns produced using a 90 nm half pitch mask were reported.2
Pitch division was achieved, but the line edge roughness of the resulting images did not meet the current standard.
To reduce line edge roughness, polymer bound PBGs and polymer bound PAGs were investigated in the PDL resist
formulations. The synthesis, purification, analysis, and functional performance of various polymers containing PBG
or PAG monomers are described herein. Both polymer bound PBG with monomeric PAG and polymer bound PAG
with monomeric PBG showed a PDL response. The performance of the polymer bound formulations is compared to
the same formulations with small molecule analogs of PAG and PBG.
We present a simple reaction rate analysis of lithographic patterning using the Non-Reciprocal Photo Base Generation
(NRPBG) scheme of Bristol (Bristol, et. al., to be published in Proceedings of the SPIE - The International Society for
Optical Engineering, 2010, presentation 7639-4). Multistep reaction kinetics simulations demonstrate that the NRPBG
scheme produces clear pitch division upon 193 nm double-exposure, over a range of photochemical reaction rate
constants.
We present an overview of lithography results achieved for materials to support "leave-on-chuck" double-exposure
pitch-division patterning. These materials attempt to make use of a non-reciprocal photoresponse in which the same
number of absorbed 193nm photons can produce different remaining levels of resist, depending upon whether the
photons are received all at once or in two separate exposures. This, in principle, allows for the use of two exposures,
using independent masks and without removing the wafer from the chuck, to produce non-regular patterning down to
one half the pitch limit of the scanner. Such behavior could be produced, for example, by a reversible two-stage
Photoacid Generator (PAG) or other non-reciprocal mechanisms.
Several stages of lithography screening were done on a large number of candidate systems. Initially, thermal stability,
casting behavior, and single-exposure (SE) contrast curves were investigated to determine whether the system behaved
as a usable photoresist. The next stage of testing probed non-reciprocal response, in the form of double-exposure (DE)
contrast curves, typically with an intervening whole-wafer flood exposure at a longer wavelength to enact the nonreciprocity.
The key criterion for the material to pass this stage was to show a shifted contrast curve (difference in
photospeed) for DE vs. SE. Such a shift would then imply that pitch-division imaging would be possible for this
material.
After identifying materials which exhibited this SE vs. DE contrast curve shift, the next step was actual DE patterning.
Since the laboratory tool used for these exposures does not have the precise alignment needed to interleave the two
exposures for pitch division, we employed a technique in which the second exposure is rotated slightly with respect to
the first exposure. This results in a Moiré-type pattern in which the two aerial images transition between overlap and
interleave across the wafer.
One particular PAG + sensitizer did indeed show the desired DE vs. SE contrast curve shift and pitch-divided imaging (k1 = 0.125). This system appears to operate on a scheme based on the creation of a photobase generator between the first and second exposures. Unfortunately, the quality of the pitch-divided images degrades quickly as the pitch is decreased, showing severe LER and bridging defects at a final pitch of 220nm. We postulate that this is caused by the diffusion of one or more key photoproducts. Accompanying papers report on both the photochemical details of the reaction pathways of these materials as well as modeling of the reaction kinetics.
A new type of scissionable polymer based on main-chain acid-labile acetal linkages is reported as a photoresist for
e-beam and EUV lithography. Four kinds of copolymers were synthesized via ring-opening metathesis polymerization
(ROMP) using various ratios of acetal and norbornene-derivative monomers. Differential scanning calorimetry (DSC)
analysis demonstrated that incorporation of bulky structure, i.e., norbornene-derivatives, provided copolymers with high
Tg. According to thermogravimetric analysis (TGA), these copolymers had slight weight loss in the temperature range
from 100 to 250°C. This weight loss is tentatively assigned to a cleavage process due to the presence of the acetal units.
Both GPC and NMR analyses revealed that the main-chain of these copolymers was steadily chopped at scission
moieties of acetal units by strong acids in solution, and was also chopped by photo-generated acid from PAG in thin-film.
A steric barrier to the scissionable moiety is considered to hinder acidolysis, leading to improvement of main-chain
stability. These copolymers were confirmed to make fine patterns by e-beam exposure, demonstrating them to be
promising materials as photoresists for EUV lithography. Significant improvements are needed to meet the required resolution and photospeed performance for incorporation into viable EUV resists.
The synthesis and characterization data for a new sulfonium photoacid generator (PAG),
diphenyltrimethylsilylmethylsulfonium triflate (I), is reported. It is shown that the molecule undergoes rapid silyl group
transfer to water or phenol in the presence of a strong, nucleophilic base such as trioctylamine (TOA). The resulting
PAG, diphenyl-methylsulfonium triflate (II), is subsequently degraded by TOA via methyl group transfer from S to N
leading to the formation of Ph2S and methyltriocylammonium triflate. Both I and II are stable when non-nucleophilic
base quenchers are used. Dose-to-clear and patterning results obtained from EUV exposures at Intel-MET are presented,
illustrating that increased sensitivity can be obtained with PAGs I and II relative to triphenylsulfonium triflate (TPSOTf),
but that LWR is compromised.
Extreme ultraviolet (EUV) lithography has gained momentum as the method of choice for <32-nm half-pitch device
fabrication. In this paper, we describe our initial attempts to increase an EUV resist's sensitivity without compromising
resolution and line roughness via introduction of a thermally crosslinkable underlayer. The main purpose is to test the
possibility of using a combination of photoacid generators (PAGs) and EUV sensitizers (phenol type) in the underlayer
designs to enhance the overall performance of EUV resists. We have demonstrated the possible benefits of adding an
EUV underlayer into the regular EUV litho stack and investigated the effect of PAG types and loadings on the
photospeed and litho performance of three different EUV resists.
We present the results of both theoretical and experimental investigations of materials for application either as a
reversible Contrast Enhancement Layer (rCEL) or a Two-Stage PAG. The purpose of these materials is to enable Litho-
Litho-Etch (LLE) patterning for Pitch Division (PD) at the 16nm logic node (2013 Manufacturing). For the rCEL, we
find from modeling using an E-M solver that such a material must posses a bleaching capability equivalent to a Dill A
parameter of greater than 100. This is at least a factor of ten greater than that achieved so far at 193nm by any usable
organic material we have tested.
In the case of the Two-Stage PAG, analytical and lithographic modeling yields a usable material process window, in
terms of reversibility and two-photon vs. one-photon acid production rates (branching ratio). One class of materials,
based on the cycloadduct of a tethered pair of anthracenes, has shown promise under testing at 193nm in acetonitrile.
Sufficient reversibility without acid production, enabled by near-UV exposure, has been achieved. Acid production as a
function of dose shows a clear quadratic component, consistent with a branching ratio greater than 1. The experimental
data also supports a acid contrast value of approximately 0.05 that could in principle be obtained with this molecule
under a pitch division double-exposure scenario.
EUV lithography (EUVL) is a leading candidate for printing sub-32 nm hp patterns. In order for EUVL to be
commercially viable at these dimensions, a continuous evolution of the photoresist material set is required to
simultaneously meet the aggressive specifications for resolution, resist sensitivity, LWR, and outgassing rate.
Alternative PAG designs, especially if tailored for EUVL, may aid in the formation of a material set that helps
achieve these aggressive targets. We describe the preparation, characterization, and lithographic evaluation of
aryl sulfonates as non-ionic or neutral photoacid generators (PAGs) for EUVL. Full lithographic
characterization is reported for our first generation resist formulation using compound H, MAP-1H-2.5. It is
benchmarked against MAP-1P-5.0, which contains the well-known sulfonium PAG, triphenylsulfonium
triflate (compound P). Z-factor analysis indicates nZ32 = 81.4 and 16.8 respectively, indicating that our first
generation aryl sulfonate formulations require about 4.8x improvement to match the results achieved with a
model onium PAG. Improving the acid generation efficiency and use of the generated byproducts is key to
the continued optimization of this class of PAGs. To that end, we believe EI-MS fragmentation patterns and
molecular simulations can be used to understand and optimize the nature and efficiency of electron-induced
PAG fragmentation.
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