Base Quencher Research Articles

Additive chemistry and distributions in photoresist thin films

The lithographic performance of thin photoresist films is a function of the distribution of formulation components, such as photoacid generator (PAG) molecules, and how these components undergo chemical modification and migrate within the film during the lithography processing steps. Argon gas cluster ion beam – secondary ion mass spectrometry depth profiles were used to monitor the PAG and quencher base distributions before and after exposure and postexposure bake processing steps for different photoresist formulations. PAG and quencher base distributions were correlated to depth of focus lithographic performance results.

Critical material properties for pattern collapse mitigation

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.

Unnatural base pair systems for sensing and diagnostic applications

Expansion of the genetic alphabet by an unnatural base pair system provides a platform for the site-specific, enzymatic incorporation of extra, functional components into nucleic acids. Recently, several unnatural base pairs that exhibit high fidelity and efficiency in PCR have been developed. Functional groups of interest, such as fluorescent dyes, can be linked to the unnatural bases, and the modified base substrates are site-specifically incorporated into nucleic acids by polymerases. Furthermore, unique unnatural base pairs between fluorophore and quencher base analogs have been developed for imaging PCR amplification and as molecular beacons. Here, we describe the recent progress in the development of unnatural base pairs that function in PCR amplification and their applications as sensing and diagnostic tools.

A New Unnatural Base Pair System between Fluorophore and Quencher Base Analogues for Nucleic Acid-Based Imaging Technology

In the development of orthogonal extra base pairs for expanding the genetic alphabet, we created novel, unnatural base pairs between fluorophore and quencher nucleobase analogues. We found that the nucleobase analogue, 2-nitropyrrole (denoted by Pn), and its 4-substitutions, such as 2-nitro-4-propynylpyrrole (Px) and 4-[3-(6-aminohexanamido)-1-propynyl]-2-nitropyrrole (NH(2)-hx-Px), act as fluorescence quenchers. The Pn and Px bases specifically pair with their pairing partner, 7-(2,2'-bithien-5-yl)imidazo[4,5-b]pyridine (Dss), which is strongly fluorescent. Thus, these unnatural Dss-Pn and Dss-Px base pairs function as reporter-quencher base pairs, and are complementarily incorporated into DNA by polymerase reactions as a third base pair in combination with the natural A-T and G-C pairs. Due to the static contact quenching, the Pn and Px quencher bases significantly decreased the fluorescence intensity of Dss by the unnatural base pairings in DNA duplexes. In addition, the Dss-Px pair exhibited high efficiency and selectivity in PCR amplification. Thus, this new unnatural base pair system would be suitable for detection methods of target nucleic acid sequences, and here we demonstrated the applications of the Dss-Pn and Dss-Px pairs as molecular beacons and in real-time PCR. The genetic alphabet expansion system with the replicable, unnatural fluorophore-quencher base pair will be a useful tool for sensing and diagnostic applications, as well as an imaging tool for basic research.

Origin of frequency-dependent line edge roughness: Monte Carlo and fast Fourier-transform studies

The low-frequency line edge roughness (LER) of a chemically amplified resist (CAR) has a marked effect on the quality of electrical circuits, especially those produced by sub-30-nm-scale fabrication by extreme ultraviolet lithography. We examined the origin of frequency-dependent LER by Monte Carlo and dissolution simulations of a positive-tone CAR subjected to electron beam lithography. The correlation between frequency components and LER is highlighted to clarify which component is dominant. We found that the resist process parameters, such as the exposure dose, the base quencher, and the development, cause low-frequency LER even in the absence of mesoscale resist roughness.

Incorporation of ionic photoacid generator (PAG) and base quencher into the resist polymer main chain for sub-50 nm resolution patterning

Previous research has shown that polymer-bound PAG resists exhibit improved lithographic performance in both 193 nm and extreme ultraviolet lithography (EUVL) applications as compared to their more traditional blended PAG analogs. The further incorporation of base directly into the resist polymer backbone is a potential route for improving the resist performance and processing window. In this work, a new series of chemically amplified resists (CARs) that incorporate both PAG and base quencher functional groups into the polymer main chain are reported. The lithographic performance of these materials was investigated using electron-beam lithography (EBL). The polymer-bound PAG cation resist showed higher photospeed than the polymer-bound PAG anion resists. Base quencher incorporation slightly decreased the photospeed but improved the resolution in the case of resists with the polymer-bound PAG cation and pyridine base. Base quencher incorporation showed no improvement on the lithographic performance of polymer-bound PAG anion resists.

Study of Acid-Base Equilibrium in Chemically Amplified Resist

Protons generated in chemically amplified resists drive pattern formation reactions such as acid catalytic deprotection reactions. Proton dynamics is controlled by the addition of base quenchers so that ultrafine patterns are obtained. However, the details of interaction between protons and base quenchers are still unclear. In this study, we investigated the reactions of protons with base quenchers in a model system of chemically amplified resists with a typical backbone polymer, poly(4-hydroxystyrene). We confirmed that an acid-base equilibrium was reached in the model system without elevating the temperature of the films. The proton affinities of resist ingredients give us a general estimation of protonation sites. When the proton affinities of resist ingredients are close to each other, we have to take into account acid-base equilibrium using pKa for the accurate prediction of protonation sites.

Measurements of acid generation by extreme ultraviolet irradiation in lithographic films

Extreme ultraviolet (EUV) lithography requires photoresist materials that incorporate highly efficient photoacid generators (PAGs) due to the low intensity of the currently available EUV light sources. It is therefore necessary to understand the parameters that control acid generation mechanisms in photoresists under EUV irradiation, such as photoacid generator and base quencher structure, polymer matrix effects, and the interaction of ionizing radiation with the resist components. In this study, a known acid detection technique based on a dye indicator (coumarin 6) has been optimized for the quantification of the amount of photoacid produced by irradiation of lithographic resist films. Incorporation of acid-base indicators to solid polymer matrices has already been employed in previous studies; however, the environmental stability of the reactive species and the influence of the polymer resin have not been considered to date. Here, the authors present a comprehensive evaluation of PAGs that were exposed to 13.5nm radiation and compared to 248nm exposures. For some specific cases studied, it was possible to correlate the acid generation efficiency to the PAG structure and polymer matrix effects. Moreover, it is found that environmental effects such as humidity have a significant impact on acid strength, acid detection, and acid generation efficiency. The acid generation efficiency experiments were complemented with dose to clear exposures for selected resist systems. The results obtained indicate that the use of acid detection methods implemented for the design of EUV resist compositions with optimized photospeed requires careful attention to the complex role of the reactivity of all formulation components and photoreaction products in the presence of ionizing radiation.

Influence of base additives on the reaction-diffusion front of model chemically amplified photoresists

The effects of amine base quencher on the photoacid catalyzed deprotection reaction-diffusion front in model photoresists were measured by combination of neutron reflectivity and Fourier transform infrared spectroscopy. Modulation in the location of the base with respect to the diffusing photoacid catalyst changes the spatial reaction extent and illuminates the complex role of the base on the shape of the reaction-diffusion front. Despite similar total extents of reaction, a comparison between uniform base and model photodegradable base distributions demonstrates distinct reaction time and base concentration effects on the deprotection profile shape. These differences arise from the modification of the initial deprotection extent due to both the neutralization of the photoacid and the influence of the changing photoresist composition on the reaction-diffusion process. The use of the model photodegradable base results in a sharper front due to these effects. Lastly, aqueous hydroxide development of these latent images demonstrates a limit to the improvement in feature quality obtained from sharpening of the deprotection profile with base additives.

Exposure dose effects on the reaction-diffusion process in model extreme ultraviolet photoresists

The effect of exposure dose on the latent image deprotection profile in a model extreme ultraviolet (EUV) photoresist polymer, poly(hydroxystyrene-co-d9-tert-butyl acrylate), is measured with neutron reflectometry. As the photoacid concentration is increased by exposure dose, the spatial extent of propagation increases but eventually becomes self-limited by the products of the reaction. A long-range deprotection path occurs with diffusion length between 10 and 100Å, while an additional subnanometer short-range deprotection length scale proceeds monotonically with dose. These measurements show that the photoacid diffusion length into unexposed regions of a photoresist is limited even in the absence of base quencher additives. These fundamental data can be used to highlight materials effects on photoresist processing and to improve quantitative models for EUV photoresists needed at the sub-32-nm half pitch lithography.

Line Edge Roughness in Chemically Amplified Resist: Speculation, Simulation and Application

Computer simulations and direct measurements were carried out to explore the effect of adding base quencher on the line edge roughness (LER) of photoresist images. The effect of variation of the polymer sequence distribution and hydrophilic units was also studied. The variation of surface roughness at different developed film thicknesses was measured by atomic force microscopy (AFM) for each polymer. It was concluded that the variation of dissolution rate (DR) and the distribution of acid and base are important parameters that must be considered in order to solve the LER problem for future lithography generation.

Low-voltage electron beam lithography resist processes: top surface imaging and hydrogen silisesquioxane bilayer

A hydrogen silisesquioxane (HSQ) bilayer process and a top surface imaging (TSI) process are investigated for application as low-voltage electron beam resist systems. Namatsu, van Delft, and others have reported printing exceptionally small features using high-voltage electron beam exposure of HSQ at high-exposure doses (~2000 µC/cm2 at 100 kV). The shallow penetration depth of low-voltage electrons results in greatly reduced dose requirements, and smooth, high-resolution images are generated at 1 kV with an exposure dose of less than 60 µC/cm2. HSQ's high silicon content enable it to be used in a bilayer form utilizing reactive ion etching with an oxygen plasma, thus generating high aspect ratio images. TSI has been studied in the past by numerous researchers at low voltages using various TSI schemes. We investigate the use of a chemically amplified TSI resist process based on poly (t-BOC-hydroxystyrene). The effect of base quencher loading in the resist formulation on line edge roughness and resolution is investigated, and is found to have a dramatic influence. High-resolution, high aspect ratio images are printed down to 40 nm, and exhibit only moderate levels of line edge roughness. Furthermore, proximity effects at 1, 2, and 3 kV are examined and compared to simulation.

The Multifunctional Role of Base Quenchers in Chemically Amplified Photoresists

A systematic investigation of four base quenchers in chemically amplified photoresist revealed that the role of the base quencher is more complex than rapid, stoichiometric neutralization of photoa...

Micromolar concentrations of base quenchers impact the apparent efficiency of photoacid generation in chemically amplified resists

The chemical composition of the base quencher added to chemically amplified photoresist formulations was found to influence the apparent efficiency of processes that lead to photoacid generation upon exposure to ionizing radiation. Resists formulated with the base quencher 1-piperidineethanol (1PE) required lower exposure doses to render development than resists containing 3-piperidino-1,2-propanediol (3P12PD) as the base. A standard addition technique was employed to determine the efficiency of photoacid generation in resist formulations consisting of one of the two base quenchers, the photoacid generator triphenylsulfonium triflate, and a matrix polymer composed of primarily p-hydroxystyrene and tert-butylacrylate monomers (environmentally stable chemically amplified photoresist type). The resist containing 1PE exhibited an apparently greater efficiency for photoacid generation than the formulation containing 3P12PD. Analysis of development processes showed that resists containing these bases exhibited identical dissolution behavior to that of a system without base quencher, verifying that resist dissolution could not explain the difference in exposure dose required for development of the 1PE and 3P12PD resists. Furthermore, the action of photoacid to deprotect the polymer, as interpreted from infrared spectroscopy, and the concentration of photoacid required to render development were the same for both resists. These results suggest that the small concentration of base in the resist may participate in the complicated chemical processes that produce photoacid from the decomposition of PAG upon exposure to ionizing radiation.

A Standard Addition Technique To Quantify Photoacid Generation in Chemically Amplified Photoresist

A technique to determine the efficiency of the decomposition of photoacid generators to produce photoacid and the concentration of acid necessary to image chemically amplified photoresists was designed, implemented, and validated. The technique is analogous to a standard addition experiment; known concentrations of a base quencher are added to a series of otherwise identical resist formulations. To produce the same free acid concentration within the resist films after neutralization, the concentration of photogenerated acid must increase by an amount equal to the known concentration of added base. The increase in exposure dose required to create this additional acid is indirectly determined as a function of base loading using resist dissolution behavior (contrast curves). The main assumptions implicit in the model to extract the Dill C parameter (a parameter proportional to the quantum yield) from experiments were validated using X-ray exposure and a positive tone photoresist system consisting of poly(p-t-butoxycarbonyloxystyrene-co-p-hydroxystyrene) as the base resin, norbornene dicarboximidyl triflate as the photoacid generator, and 1-piperidineethanol as the base quencher. Resin deprotection monitored by FTIR and dissolution rate measurements provided independent evidence that the bulk dissolution rate, the dissolution induction time, and the extent of deprotection as a function of the free acid concentration remained the same for resists with and without base.