Ultrathin Resists Research Articles

Impact of vacuum ultraviolet photons on ultrathin polymethylmethacrylate during plasma etching

State-of-the-art extreme ultraviolet lithography requires the use of ultrathin photoresists (or resists) due to pattern stability concerns and reduced depth of focus of the extreme ultraviolet lithography scanners. Current resists for extreme ultraviolet lithography are less than 50 nm thick. These ultrathin resists further complicate pattern transfer as unintended plasma-induced damage during dry etching is more pronounced. A better understanding of the interaction of plasma species with ultrathin resists is critical for enabling pattern transfer of sub-10 nm features. Here, we study the impact of vacuum ultraviolet photons, argon ions, and argon plasma on a 40 nm thick polymethylmethacrylate film. Using a deuterium lamp, an industrial ion beam etch tool, and an industrial inductively coupled plasma etch tool, we exposed the polymer to photons, ions, and plasma, respectively. The exposed samples were then analyzed for chemical and physical changes using different characterization techniques. It was observed that the vacuum ultraviolet photons interact with the entire bulk of polymer film, while the ions only affect the surface and subsurface region. The photon exposed samples formed smaller polymer fragments at low exposure doses and further started to cross-link at high doses. In contrast, the ion modification leads to carbonization of only the top few nanometers of the polymer film, leaving the bottom bulk intact. The plasma exposed sample showed changes characteristic to both vacuum ultraviolet photons and ions and their synergism. It was stratified with a 1.34 ± 0.03 nm thick ion-caused carbonized layer on top of a 13.25 ± 0.12 nm photon-induced cross-linked layer. By studying the impact of plasma photons on ultrathin polymethylmethacrylate, we were able to establish a baseline for a testing methodology that can be extended to novel ultrathin resist platforms.

Interfacial Electron Beam Lithography: Chemical Monolayer Nanopatterning via Electron-Beam-Induced Interfacial Solid-Phase Oxidation.

Chemical nanopatterning-the deliberate nanoscale modification of the chemical nature of a solid surface-is conveniently realized using organic monolayer coatings to impart well-defined chemical functionalities to selected surface regions of the coated solid. Most monolayer patterning methods, however, exploit destructive processes that introduce topographic as well as other undesired structural and chemical transformations along with the desired surface chemical modification. In particular in electron beam lithography (EBL), organic monolayers have been used mainly as ultrathin resists capable of improving the resolution of patterning via local deposition or removal of material. On the basis of the recent discovery of a class of radiation-induced interfacial chemical transformations confined to the contact surface between two solids, we have advanced a direct, nondestructive EBL approach to chemical nanopatterning-interfacial electron beam lithography (IEBL)-demonstrated here by the e-beam-induced local oxidation of the -CH3 surface moieties of a highly ordered self-assembled n-alkylsilane monolayer to -COOH while fully preserving the monolayer structural integrity and molecular organization. In this conceptually different EBL process, the traditional resist is replaced by a thin film coating that acts as a site-activated reagent/catalyst in the chemical modification of the coated surface, here the top surface of the to-be-patterned monolayer. Structural and chemical transformations induced in the thin film coating and the underlying monolayer upon exposure to the electron beam were elucidated using a semiquantitative surface characterization methodology that combines multimode AFM imaging with postpatterning surface chemical modifications and quantitative micro-FTIR measurements. IEBL offers attractive opportunities in chemical nanopatterning, for example, by enabling the application of the advanced EBL technology to the straightforward nanoscale functionalization of the simplest commonly used organosilane monolayers.

In situ Dissolution Analysis of Ultrathin Extreme Ultraviolet Resists

With extreme ultraviolet (EUV) resists reaching resolution capabilities beyond the 20 nm half-pitch line and space patterns, a trend has emerged in applying ultrathin resists (film thickness less than 60 nm) to compensate for possible issues in resolution-limiting pattern collapse and line width roughness, among other aspects. Thus, a clear understanding of the pattern mechanisms of resists at these extreme conditions is necessary. This paper focuses on the pattern formation characteristics of various EUV resist platforms for ultrathin films during dissolution by using in situ high-speed atomic force microscopy in liquid. As a result, it was observed that, regardless of the resist platform, there is a decrease in the size of the “dissolution clusters” at ultrathin conditions. Such fundamental information on ultrathin resist films may provide possible pointers to further extend the potential of EUV resists for achieving higher resolution limits and minimal line width roughness.

Ultra-Thin Resists for Advanced Lithography

The continuous reduction of feature sizes in integrated circuits (IC) is driving the development of advanced lithographic techniques that enable patterning of sub-100-nm features with good fidelity and process margins. Requirements for printing these sub-100-nm features includes tight control of critical dimensions (CD), line edge roughness, and resist profiles. Reduction in exposure wavelengths to 157 nm and extreme ultraviolet (EUV) is pushing the development of new families of resist systems and the use of thin imaging layers that overcome the high absorption by materials of 157-nm and EUV radiation. In addition, thin imaging techniques may provide benefits for low-k1 and high-NA lithography using 193-nm radiation. The use of ultra-thin resists (UTR) introduces new concerns for resist properties including the possibility for increased defectivity and the reduction in etch resistance. In this paper, we describe results of studies using ultra-thin resist integrations to build working devices. These investigations have shown that UTR are not inherently limited by defectivity, and working devices with high yield have been fabricated using UTR integrations on multiple levels.

Ultrathin Resist Pattern Transfer Process by Filling Mask Material in the Resist Pattern

We performed experiments to transfer the pattern of ultrathin resists with the reversal mask process. Resist film thickness was 100 nm for samples using the KrF resist (KrF sample) and the ArF resist (ArF sample), and 70 nm for the sample using the EB resist (EB sample). Resist patterns were formed on an organic film, and then cured by EB. Spin-on-glass (SOG) was coated over the resist patterns and etched back. Finally, we obtained organic film patterns using the remaining SOG as an etch mask. In the case of the KrF sample, it was possible to transfer three resist patterns (lines and spaces (LS), isolated line (iL), isolated space (iS)) to the organic film simultaneously. In case of the ArF sample and the EB sample, it was possible to transfer each pattern. However, the transfers were not simultaneous. In order to transfer different patterns simultaneously, strengthening etch resistance of mask material, improving uniformity of SOG thickness over the resist pattern and suppression of shrinkage of resist pattern thickness caused by EB cure are important to achieve.

Prospects for photolithography at 121 nm

We have evaluated several subsystems that would be required for lithography at 121 nm: radiation sources, optics, ambient control, and resists. Dense plasma sources can generate significant power, >0.5 J of 121 nm radiation per pulse. An extensive survey of commercially available LiF, as well as specially grown LiF, shows that transmission losses are ∼20% per cm, a value which is at least 5× too high if LiF is to be used as lens material. At present it is not clear whether there is a fundamental limitation to LiF transparency at 121 nm, but the main source of absorption arises from lithium oxide or hydroxide in the crystal. Polymers, fluorinated or not, exhibit very high absorption at 121 nm, indicating that ultrathin resists or bilayer resists will have to be used at 121 nm. However, the photochemistry at 121 nm is probably not much different from that at longer wavelengths. The resists are sensitive, ∼1 mJ/cm2 required dose, and they have high contrast at 121 nm. Finally, we have also performed preliminary patterning experiments. Using contact printing with a phase shifting mask we have printed 45 nm features in an ultrathin photoresist.

Scanning Tunneling Spectroscopy of Insulating Self-Assembled Monolayers on Au(111)

Self-assembled monolayers (SAMs) of dodecanethiol (DDT), octadecanethiol (ODT), and resorcinarene C10 tetrasulfide (RC10TS) on Au(111) were characterized by scanning tunneling spectroscopy (STS) under ultrahigh vacuum conditions and evaluated as ultrathin resists. A voltage division factor η was used to parametrize the equilibrium Fermi energy level Ef in terms of an applied voltage bias. Nominally symmetric tunneling (η ∼ 0.5) conditions were established by acquiring conductance spectra over a range of set point voltages. The electrical conductivity of the monolayers was observed to be dependent on both the monolayer thickness and the nature of the Au/S bonding. Leakage current densities across DDT and RC10TS SAMs on Au were estimated, with the latter comparing favorably with a 1.5-nm layer of SiO2 on Si.

Defect-Tolerant and Directional Wet-Etch Systems for Using Monolayers as Resists

We developed strategies based on self-assembly principles to etch substrates patterned with monolayer resists with high selectivity and etch directionality. Our strategies exploit the defined composition and order of these ultrathin resists and overcome their imperfections. Defects in a monolayer can be healed by additives present in an etch bath. Alternatively, large molecules that cannot diffuse through defects can be employed as etchants. It is also possible to taper structures using the competition between etching and the side-growth of a self-assembling etch barrier, nucleating from the originally patterned monolayer. The application of these concepts lets defect-sensitive monolayers become robust and versatile resists, which should promote their acceptance and use in microtechnology.

Organized molecular assemblies for scanning probe microscope lithography

Abstract Nanostructures down to a few ten nanometers in size have been fabricated with Langmuir–Blodgett (LB) films and self-assembled monolayers (SAMs) using scanning probe microscope lithography. The SAMs have been prepared with organosilane and bipolar amphiphiles, alkanethiol molecules as ultrathin resists on Si and Au substrates. The LB films on silicon substrates using both the polymer of thiophene derivatives and the mixture of palmitic acid and hexadecylamine were prepared and fabricated. The effect of functional groups of molecules on the atomic force microscope (AFM) anodization has been studied in the optimized process conditions. Applied voltage between the AFM tip and sample, the scanning speed and the relative humidity in the laboratory are also important factors for nanometer-scale lithography of the ultrathin films. The STM lithography with an octadecanethiol SAM on Au film in the air was carried out at the pulse mode of tip bias with respect to the suitability of STM lithography. The high structural orderness and perfect thickness of ultrathin organic molecular assemblies are the major advantages as required for nanoscale lithography.

Ultra Thin Film Resist for Low Energy E-beam Projection Lithography.

Recently, new resist criteria for low energy electron beam projection lithography (LEEPL) such as bi-layer, chemically amplified and silicon containing resists have been requested. In order to investigate the lithographic characteristics of each system, three chemically amplified resist samples consisting of silsesquioxane base polymer were prepared. It was found using a 50kV direct writing electron beam tool that the resist characteristics at 50nm film thickness indicated unique results compared with that of the 100nm, 150nm and 200nm film thickness in the resist screening. The screened resists presented around 80nm resolution with 180nm pitch in LEEPL imaging. However, the coating defect was detailed as a challenge for imaging ultra thin film resists.

STM Lithography in an Organic Self-Assembled Monolayer

We describe the suitability of ultra-high vacuum scanning tunneling microscopy (UHV-STM) based nanolithography by using highly ordered monomolecular organic films, called self-assembled monolayers (SAMs), as ultrathin resists. Organothiol-type SAMs such as hexadecanethiol (SH–(CH2)15–CH3) and N-biphenylthiol (SH–(C6H6)2–NO2) monolayers have been prepared by immersion on gold films and Au(111) single crystals. Organosilane-type SAMs such as octadecyltrichlorosilane (SiCl3–(CH2)17–CH3) monolayers have been prepared on hydroxylated Si(100) surfaces as well as hydroxylated chromium film surfaces. Dense line patterns have been written by UHV-STM in constant current mode for various tunneling parameters (gap voltage, tunneling current, scan speed, and orientation) and transferred into the underlying substrate by wet etch techniques. The etched structures have been analyzed by means of scanning electron microscopy (SEM) and atomic force microscopy (AFM). Best resolution has been achieved without etch transfer for a 20 nm × 20 nm square written in hexadecanethiol/Au(111) with an edge definition of about 5 nm. Etch transfer of the STM nanopatterns in Au films resulted in 55 nm dense line patterns (15 nm deep) mainly broadened by the isotropic etch characteristic, while 35 nm wide and 30 nm deep dense line patterns written in octadecyltrichlorosilane/Si(100) and anisotropically etched into Si(100) could be achieved.

STM Lithography in an Organic Self-Assembled Monolayer

We describe the suitability of ultra-high vacuum scanning tunneling microscopy (UHV-STM) based nanolithography by using highly ordered monomolecular organic films, called self-assembled monolayers (SAMs), as ultrathin resists. Organothiol-type SAMs such as hexadecanethiol (SH–(CH2)15–CH3) and N-biphenylthiol (SH–(C6H6)2–NO2) monolayers have been prepared by immersion on gold films and Au(111) single crystals. Organosilane-type SAMs such as octadecyltrichlorosilane (SiCl3–(CH2)17–CH3) monolayers have been prepared on hydroxylated Si(100) surfaces as well as hydroxylated chromium film surfaces. Dense line patterns have been written by UHV-STM in constant current mode for various tunneling parameters (gap voltage, tunneling current, scan speed, and orientation) and transferred into the underlying substrate by wet etch techniques. The etched structures have been analyzed by means of scanning electron microscopy (SEM) and atomic force microscopy (AFM). Best resolution has been achieved without etch transfer for a 20 nm × 20 nm square written in hexadecanethiol/Au(111) with an edge definition of about 5 nm. Etch transfer of the STM nanopatterns in Au films resulted in 55 nm dense line patterns (15 nm deep) mainly broadened by the isotropic etch characteristic, while 35 nm wide and 30 nm deep dense line patterns written in octadecyltrichlorosilane/Si(100) and anisotropically etched into Si(100) could be achieved.

The thermal chemistry of model organosulfur compounds on gallium arsenide (110)

Organothiols and related organosulfur compounds hold promise for use as self-assembled ultra-thin electron beam resists, II–VI material growth precursors, and monolayer passivation layers. We report the results of temperature-programmed reaction and desorption (TPR/D) studies of the surface chemistry of three model organosulfur compounds, CH 3SH, (CH 3S) 2 and (CH 3) 2S, on the (110) face of gallium arsenide. Our measurements indicate that each of these species interacts strongly with the GaAs(110) surface. The first monolayer of CH 3SH desorbs at ∼300 K, with a desorption wave peak temperature (extrapolated to the zero coverage limit) corresponding to a desorption activation energy of 0.81 eV. This value is high compared to ∼110 K for the second layer, suggesting a chemical interaction between the thiol and the surface, despite the fact that the monolayer feature in the TPR/D spectra exhibits first-order kinetics, and the molecule is observed to desorb intact. The behavior of (CH 3S) 2 is markedly different; the disulfide decomposes upon adsorption (or during the TPR/D temperature ramp) and desorbs predominantly as (CH 3) 2S at ∼500 K, perhaps as the result of a concerted associative/dissociative desorption process that leaves sulfur at the surface. This result is also evidence that adsorbed CH 3SH and (CH 3) 2S do not assume equivalent surface-bound forms and supports the idea that the hydrogen atom of the thiol remains in close association with the adsorbed parent molecule, while the disulfide undergoes dissociative adsorption. Finally, our results indicate that (CH 3) 2S, like CH 3SH, molecularly adsorbs/desorbs. The desorption activation energy, extrapolated to zero coverage, is 0.79 eV, consistent with a strong adsorbate–substrate interaction. The similarity of this value to that for CH 3SH suggests that a specific sulfur–surface interaction dominates the desorption kinetics at low coverage. In the monolayer regime, with increasing exposure, the monolayer desorption feature in the TPR/D spectra broadens and exhibits strong asymmetry, and ultimately develops a lower temperature feature, indicating complex kinetics consistent with coverage-dependent phase behavior. Finally, by comparison of the measured desorption energies of the organosulfur compounds with those of the alkyl halides, for which the adsorption on GaAs(110) has been extensively investigated, an estimation of the chemical and physical contributions to the molecule–surface interaction is obtained. Our analysis suggests that both types of interactions play important roles in the adsorption energetics.

Resists and Other Critical Issues in 157-nm Lithography.

This paper outlines the critical issues facing the implementation of 157-nm lithography as a sub-100-nm technology. Special emphasis is placed on 157-nm photoresist options, including ultrathin resists and thicker resists with new chemistries. The status of other issues is also presented, such as mask materials, pellicles, optical materials, and coatings.

Comparison of the lithographic properties of positive resists upon exposure to deep- and extreme-ultraviolet radiation

Nineteen chemically amplified ultrathin resists were imaged using exposure to extreme-ultraviolet (EUV) (13.4 nm) and deep-ultraviolet (DUV) (248 nm) radiation. Direct comparisons were made of photospeed, resolution, and line edge roughness (LER). The photospeed of these resists at 248 nm shows a good correlation with photospeed at EUV for three polymer types, but appears independent of photoacid generator type. This result underscores the importance of the polymer in photoacid generation at EUV. Resolution showed poor correlation between DUV and EUV. Correlations were made between the line edge roughness of EUV-imaged features and unexposed film thickness loss, resist contrast, image log slope (ILS), and LER of resists exposed at DUV. Both contrast and image log slope play important roles in defining LER performance—where the best LER is achieved at high contrast and high ILS.

Pattern transfer: Self-assembled monolayers as ultrathin resists

This review includes three sections: (i) preparation, structure, and properties of self-assembled monolayers (SAMs); (ii) techniques for patterning SAMs, including microcontact printing (μCP), UV-photolithography, and e-beam writing; and (iii) use of patterned SAMs as ultrathin resists (2–3 nm thick) in processes for pattern transfer based on selective chemical etching and selective deposition. Microcontact printing is a non-lithographic technique for forming patterned features with dimensions ≧100 nm; the initial product of patterning is organized monolayers of alkanethiolates on Au, Ag, Cu and GaAs, and of alkylsiloxanes on Si/SiO 2 and glass. In this technique, an elastomeric stamp having a surface patterned with a relief structure is used to generate patterned SAMs on the surfaces of solid materials. These patterned SAMs are resists that protect the underlying substrates from dissolution in selective etchants (for example, for evaporated thin films of Au and Ag, aqueous solutions of K 2S 2O 3, K 3Fe(CN) 6 and K 4Fe(CN) 6). Patterned structures of gold or silver that are produced by the combination of μCP and selective etching can be used as the secondary masks for subsequent processes such as isotropic etching of SiO 2, isotropic or anisotropic etching of Si, anisotropic etching of GaAs, and reactive ion etching (RIE) of Si. Patterned SAMs can also be used as templates for selective deposition of metals by chemical vapor deposition (CVD), electroplating, or electroless deposition.

Imaging and modification of polymers by scanning tunneling and atomic force microscopy

Direct imaging of ultrathin organic films on solid surfaces is important for a variety of reasons; in particular, the use of such films as ultrathin resists for nanometer scale fabrication and information recording requires that we understand their microstrucure. We have used the Langmuir–Blodgett technique to prepare monolayer and submonolayer films of poly(octadecylacrylate) (PODA) and poly(methylmethacrylate) (PMMA) on graphite substrates. Atomic scale images obtained with the scanning tunneling microscope (STM) and the atomic force microscope of the PODA films showed a variety of structures, including isolated narrow fibrils, parallel groups of fibrils, and an ordered structure consistent with the side chain crystallization expected with that material. The fibrils observed are interpreted as individual polymer chains or small bundles of parallel chains. Images of the PMMA samples show no ordered regions. By applying voltage pulses on the STM tip, we were able to locally modify and apparently cut through the PODA fibrils.

Drawing fine patterns on N-octadecylacrylamide Langmuir-Blodgett multilayers: a new class of ultrathin resists

Drawing fine patterns on N-octadecylacrylamide Langmuir-Blodgett multilayers: a new class of ultrathin resists