Electron Beam Resists Research Articles

In-situ imaging of heat-induced phase transition in a two-dimensional conjugated metal-organic framework

Atomically-resolved in-situ high-resolution transmission electron microscopy (HRTEM) imaging of the structural dynamics in organic materials remains a major challenge. This difficulty persists even with aberration-corrected instruments, as HRTEM images necessitate a high electron dose that is generally intolerable for organic materials. In this study, we report the in-situ HRTEM imaging of heat-induced structural dynamics in a benzenehexathiol-based two-dimensional conjugated metal-organic framework (2D c-MOF, i.e., Cu3(BHT)). Leveraging its hydrogen-free structure and high electrical conductivity, Cu3(BHT) exhibits high electron beam resistance. We demonstrate atomic resolution imaging at an 80 kV electron accelerating voltage using our Cc/Cs-corrected SALVE instrument. However, continuous electron irradiation eventually leads to its amorphization. Intriguingly, under heating in a MEMS holder, the Cu3(BHT) undergoes a phase transition to a new crystalline phase and its phase transition, occurring within the temperature range of 480 °C to 620 °C in dependence on the electron beam illumination. Using HRTEM and energy-dispersive X-ray mapping, we identify this new phase as CuS. Our findings provide insights into the mechanisms governing structural transitions in purposefully engineered structures, potentially pivotal for future endeavours involving the production of metal oxide/sulfide nanoparticles from MOF precursors.

Mitigating challenges in aberration-corrected electron-beam lithography on electron-opaque substrates

Aberration-corrected electron-beam lithography (AC-EBL) using ultra-thin electron transparent membranes has achieved single-digit nanometer resolution in two widely used electron-beam resists: poly (methyl methacrylate) (PMMA) and hydrogen silsesquioxane. On the other hand, AC-EBL implementation on thick, electron-opaque substrates is appealing for conventional top-down fabrication of quantum devices with nanometer-scale features. To investigate the performance of AC-EBL on thick substrates, we measured the lithographic point spread function of a 200 keV aberration-corrected scanning transmission electron microscope by defining both positive and negative patterns in PMMA thin films, spin-cast on thick SiO2/Si substrates. We present the problems encountered during pre-exposure beam focusing and discuss methods to overcome them. In addition, applying some of these methods using commercial 50 nm thick SiN X membranes with thick Si support frames, we printed arrays of holes in PMMA with pitches around 26 nm on SiN X /Si substrates with increasing Si thickness. Our results show that proximity effects from even 50 nm thick SiN X membranes limit hole arrays to 20 nm pitch; however, down to this limit, the effect of the substrate thickness on the pattern quality is minimal. These results highlight the need for novel resists less susceptible to proximity effects, or resists which can be used directly, after development, as the dielectric material in periodic gates in 2D quantum devices.

Study and comparison of resist characteristics for different negative tone electron beam resists

In this work, four types of negative electron beam resists are investigated. Electron beam lithography (EBL) experiments were conducted using EBL system ZBA23 (Raith) with the variable-shaped electron beam cross-section at 40 keV electron energy. Important electron beam resist characteristics such as sensitivity, dissolution rate, aspect ratio and sidewall developed profiles in the chemically amplified resist (CAR) SU-8 2000, non-CARs ma-N 2410 and ARN-7520, and inorganic negative resist HSQ XR-1514 are studied and compared. This study was motivated by the selection of a suitable resist for practical use such as large area gratings fabrication for optoelectronics.

Focused-Ion-Beam Exposure of an Ultrathin Electron-Beam Resist for the Formation of Nanoscale Field-Effect Transistor Contacts

Focused-Ion-Beam Exposure of an Ultrathin Electron-Beam Resist for the Formation of Nanoscale Field-Effect Transistor Contacts

Molecular Layer Deposition of a Hafnium-Based Hybrid Thin Film as an Electron Beam Resist.

The development of new resist materials is vital to fabrication techniques for next-generation microelectronics. Inorganic resists are promising candidates because they have higher etch resistance, are more impervious to pattern collapse, and are more absorbing of extreme ultraviolet (EUV) radiation than organic resists. However, there is limited understanding about how they behave under irradiation. In this work, a Hf-based hybrid thin film resist, known as "hafnicone", is deposited from the vapor-phase via molecular layer deposition (MLD), and its electron-beam and deep-ultraviolet (DUV)-induced patterning mechanism is explored. The hafnicone thin films are deposited at 100 °C by using the Hf precursor tetrakis(dimethylamido)hafnium(IV) and the organic precursor ethylene glycol. E-beam lithography, scanning electron microscopy, and profilometry are used to investigate the resist performance of hafnicone. With 3 M HCl as the developer, hafnicone behaves as a negative tone resist which exhibits a sensitivity of 400 μC/cm2 and the ability to resolve 50 nm line widths. The resist is characterized via X-ray photoelectron spectroscopy (XPS) and infrared spectroscopy (IR) to investigate the patterning mechanism, which is described in the context of classical nucleation theory. This study of hafnicone hybrid MLD demonstrates the ability for the bottom-up vapor deposition of inorganic resists to be utilized in advanced e-beam and DUV lithographic techniques.

Relationship between blurring factors and interfacial effects in chemically amplified resist processes in photomask fabrication

Chemically amplified resists are an indispensable technology for photomask fabrication. The formation of latent images near interfaces in resist films is strongly affected by low-energy electron dynamics at the interfaces. On the other hand, the latent images are blurred by low-energy electron migration and acid diffusion in latent image formation. In this study, the relationship between blurring factors and interfacial effects was investigated by simulating the latent image formation on the basis of the sensitization and reaction mechanisms of chemically amplified electron beam resists. For low-energy electron dynamics, the interfacial effects on the protected unit distribution was reduced and the chemical gradient slightly increased with decreasing thermalization distance. For acid diffusion, the decrease in exposure pattern width significantly reduced the interfacial effects on the protected unit distribution in the depth direction because the acid diffusion length can be increased without sacrificing resolution.

Optimization of Development Process for Carbon Dioxide-Based Poly(cyclohexene carbonate) Electron Beam Resist

Optimization of Development Process for Carbon Dioxide-Based Poly(cyclohexene carbonate) Electron Beam Resist

Changes in molecular weight distribution caused by main-chain scission of electron beam resists

Main-chain-scission-type resists have attracted much attention as the next-generation electron beam (EB) resist used for photomask fabrication. In this study, the gel permeation chromatography (GPC) charts of irradiated EB resists (ZEP series of ZEON) were analyzed by simulation to clarify the changes in the molecular weight distribution caused by the main-chain scission of a resist polymer. In the low-exposure-dose region, the molecular weight distribution obtained by GPC was well reproduced by the simulation using a random-scission model. In the high-exposure-dose region, the calculated molecular weight distribution deviated from the experimental one because of crosslinking and the generation of stable components.

CO2‐Based Dual‐Tone Resists for Electron Beam Lithography

AbstractThe increasing global environmental and energy crisis has urgently motivated the advancement of sustainable materials. Significant effort has been focused on developing new materials to replace the fossil‐based resists in the semiconductor industry based on greener sources such as ice, dry ice, small organic molecules, and proteins. Such resist materials, however, have yet to meet the stringent requirements of high sensitivity, high resolution, reliable repeatability, and good compatibility with the current protocols. To this end, CO2‐based polycarbonates (CO2‐PCs) obtained from the copolymerization of CO2 and epoxides are demonstrated as sustainable dual‐tone (positive & negative tone) resists for electron beam lithography. By adjusting the chemical structure, developing agent, and molecular weight, the CO2‐PCs present high sensitivities to electron beam (1.3/120 µC cm−2), narrow critical dimensions (29/58 nm), and moderate line edge roughness (4.6/26.7 nm) for negative and positive resists, respectively. A deep understanding of the exposure mechanism of CO2‐PC resists is provided on the basis of the Fourier transform infrared, Raman, and electron ionization mass spectroscopy. 2D photonic crystal devices are fabricated using the negative and positive CO2‐PC resists, respectively, and both devices show distinct colors derived from their well‐defined nanostructures, indicating the great practical potential of CO2‐derived electron beam resists.

Molecular beam epitaxy of the magnetic Kagome metal FeSn on LaAlO3 (111)

Materials with Kagome layers are expected to give rise to rich physics arising from band structures with topological properties, spin liquid behavior, and the formation of Skyrmions. Until now, most work on Kagome materials has been performed on bulk samples due to difficulties in thin film synthesis. Here, by using molecular beam epitaxy, layered Kagome-structured FeSn films are synthesized on the (111) oriented LaAlO3 substrate. Both in situ and ex situ characterizations indicate that these films are highly crystalline and c-axis oriented, with atomically smooth surfaces. The films grow as disconnected islands, with lateral dimensions on the micron meter scale. By patterning Pt electrodes using a focused electron beam, the longitudinal and transverse resistance of single islands have been measured in magnetic fields. Our work opens a pathway for exploring mesoscale transport properties in thin films of Kagome materials and related devices.

Magnetoresistance effect in a vertical spin valve fabricated with a dry-transferred CVD graphene and a resist-free process

One of the most prominent and effective applications of graphene in the field of spintronics is its use as a spacer layer between ferromagnetic metals in vertical spin valve devices, which are widely used as magnetic sensors. The magnetoresistance in such devices can be enhanced by a selection of suitable spacer materials and proper fabrication procedures. Here, we report the use of dry-transferred single- and double-layer graphene, grown by chemical vapor deposition (CVD), as the spacer layer and the fabrication procedure in which no photo-resist or electron-beam resists is used. The measured maximum magnetoresistance of NiFe/CVD-Graphene/Co junction is 0.9% for the single- and 1.2% for the double-layer graphene at 30 K. The spin polarization efficiency of the ferromagnetic electrodes is about 6.7% and 8% for the single- and the double-layer graphene, respectively, at the same temperature. The bias-independent magnetoresistance rules out any contamination and oxidation of the interfaces between the ferromagnet and the graphene. The magnetoresistance measured as a function of tilted magnetic field at different angles showed no changes in the maximum value, which implies that the magnetoresistance signal is absent from anisotropic effects.

Theoretical study on protected unit fluctuation of chemically amplified resists used for photomask production by electron beam lithography

Stochasticity in the sensitization processes of resist materials is a significant concern in fine patterning. Such stochasticity results in protected unit fluctuation in the case of a chemically amplified resist, which is a standard resist for photomask production. After development, protected unit fluctuation comes to the surface as line edge roughness, stochastic pinching, and stochastic bridging. In this study, protected unit fluctuation after postexposure baking was investigated, assuming the fabrication of line-and-space patterns. The standard deviation of the number of protected units connected to a polymer molecule was calculated using a Monte Carlo method on the basis of the sensitization mechanism of chemically amplified electron beam resists. The dependences of the standard deviation on half-pitch, sensitivity, sensitizer concentration, and exposure pattern width were clarified.

Theoretical study on effects of electron thermal energy on sensitization process of chemically amplified electron beam resists—contribution to resist heating effect in electron beam mask writing

The elevation of resist temperature during exposure to electron beam (EB) deforms the patterns of chemically amplified resists. The resist heating effect is a significant problem in mask production using a high-current EB mask writer. However, its mechanism is still unknown. In this study, the effects of electron thermal energy on the sensitization process of chemically amplified EB resists were investigated using simulation on the basis of their sensitization mechanism. The decomposition yield of sensitizers increased with resist temperature. The effect of the increase of decomposition yield on the critical dimension (CD) was calculated on the basis of the reaction mechanism. When the exposure pattern width was set to the same as the half-pitch (16 nm), the values of dCD/dT were 0.019, 0.013 and 0.010 nm K−1 for the sensitizer concentrations of 0.2, 0.3 and 0.4 (molecules) nm−3, respectively. dCD/dT was decreased by decreasing the exposure pattern width.

Theoretical study on trade-off relationships between resolution, line edge roughness, and sensitivity in photomask production by electron beam lithography

The trade-off relationships between resolution, line edge roughness (LER), and sensitivity have been intensively investigated for the resist materials and processes used for extreme ultraviolet lithography. However, their relationships of the resist materials and processes in the photomask production using the electron beam (EB) lithography have not been investigated in details. In this study, the dependences of chemical gradient on the half-pitch of line-and-space patterns, exposure pattern width, and sensitizer concentration were investigated using a simulation on the basis of the sensitization and reaction mechanisms of chemically amplified EB resists. The chemical gradient is an indicator of LER. The relationships between resolution, LER, and sensitivity were formulated.

Single-Digit Nanometer Electron-Beam Lithography with an Aberration-Corrected Scanning Transmission Electron Microscope

We demonstrate extension of electron-beam lithography using conventional resists and pattern transfer processes to single-digit nanometer dimensions by employing an aberration-corrected scanning transmission electron microscope as the exposure tool. Here, we present results of single-digit nanometer patterning of two widely used electron-beam resists: poly (methyl methacrylate) and hydrogen silsesquioxane. The method achieves sub-5 nanometer features in poly (methyl methacrylate) and sub-10 nanometer resolution in hydrogen silsesquioxane. High-fidelity transfer of these patterns into target materials of choice can be performed using metal lift-off, plasma etch, and resist infiltration with organometallics.

Single-Digit Nanometer Electron-Beam Lithography with an Aberration-Corrected Scanning Transmission Electron Microscope.

We demonstrate extension of electron-beam lithography using conventional resists and pattern transfer processes to single-digit nanometer dimensions by employing an aberration-corrected scanning transmission electron microscope as the exposure tool. Here, we present results of single-digit nanometer patterning of two widely used electron-beam resists: poly (methyl methacrylate) and hydrogen silsesquioxane. The method achieves sub-5 nanometer features in poly (methyl methacrylate) and sub-10 nanometer resolution in hydrogen silsesquioxane. High-fidelity transfer of these patterns into target materials of choice can be performed using metal lift-off, plasma etch, and resist infiltration with organometallics.

Elemental depth profiles and plasma etching rates of positive-tone electron beam resists after sequential infiltration synthesis of alumina

By scanning transmission electron microscopy and energy dispersive X-ray spectroscopy (STEM–EDS), we investigated the elemental depth profiles of organic electron beam resist films after the sequential infiltration synthesis (SIS) of inorganic alumina. Although a 40-nm-thick poly(methyl methacrylate) (PMMA) film was entirely hybridized with alumina, an uneven distribution was observed near the interface between the substrate and the resist as well as near the resist surface. The uneven distribution was observed around the center of a 100-nm-thick PMMA film. The thicknesses of the PMMA and CSAR62 resist films decreased almost linearly as functions of plasma etching period. The comparison of etching rate among oxygen reactive ion etching, C3F8 reactive ion beam etching (RIBE), and Ar ion beam milling suggested that the SIS treatment enhanced the etching resistance of the electron beam resists to chemical reactions rather than to ion collisions. We proposed oxygen- and Ar-assisted C3F8 RIBE for the fabrication of silica imprint molds by electron beam lithography.

Short Review on Fullerene-Containing Electrically Conducting Electron Beam Resist for Organic Biosensors with Nanostructures

This paper provides an overview of nanocomposite organic electron beam (EB) resist polymers from the viewpoint of advanced techniques for biology and medicine. An outstanding issue with organic devices is the difficulty of simultaneously controlling the lateral size and position of structures at submicron or nanometer scales. Nanocomposite EB organic resist of ZEP520a containing [6,6]-phenyl-C61 butyric acid methyl ester (PCBM) is an excellent electrically conductive material for nanometer lateral-scale organic electronic devices. The distribution of PCBM aggregations in the nanocomposite is examined. The results open the door to the simple fabrication of densely integrated sensitive biosensors with electrical nanowires and single-electron transistors for multiplexed and simultaneous diagnoses.

Theoretical study on effects of exposure pattern width on line edge roughness and stochastic defect generation in fabrication of 16-nm-half-pitch line-and-space patterns by electron beam lithography

The resolution of electron beam (EB) mask writers has reached a 16 nm half-pitch. The suppression of stochastic phenomena such as line edge roughness (LER) and stochastic defect generation is an important issue in the high-resolution fabrication of photomasks and nanoimprint molds. In this study, the effects of exposure pattern width on LER and stochastic defect generation were theoretically investigated on the basis of the sensitization and reaction mechanisms of chemically amplified EB resists. The exposure pattern width affected stochastic bridging more than stochastic pinching and LER. The optimum pattern widths for the suppression of stochastic bridging in the fabrication of 16-nm-half-pitch line-and-space patterns were 8, 8, 10, and 12 nm for the sensitivities of 300, 240, 180, and 120 µC cm−2, respectively.

Mapping Photoemission and Hot-Electron Emission from Plasmonic Nanoantennas.

Understanding plasmon-mediated electron emission and energy transfer on the nanometer length scale is critical to controlling light-matter interactions at nanoscale dimensions. In a high-resolution lithographic material, electron emission and energy transfer lead to chemical transformations. In this work, we employ such chemical transformations in two different high-resolution electron-beam lithography resists, poly(methyl methacrylate) (PMMA) and hydrogen silsesquioxane (HSQ), to map local electron emission and energy transfer with nanometer resolution from plasmonic nanoantennas excited by femtosecond laser pulses. We observe exposure of the electron-beam resists (both PMMA and HSQ) in regions on the surface of nanoantennas where the local field is significantly enhanced. Exposure in these regions is consistent with previously reported optical-field-controlled electron emission from plasmonic hotspots as well as earlier work on low-electron-energy scanning probe lithography. For HSQ, in addition to exposure in hotspots, we observe resist exposure at the centers of rod-shaped nanoantennas in addition to exposure in plasmonic hotspots. Optical field enhancement is minimized at the center of nanorods suggesting that exposure in these regions involves a different mechanism to that in plasmonic hotspots. Our simulations suggest that exposure at the center of nanorods results from the emission of hot electrons produced via plasmon decay in the nanorods. Overall, the results presented in this work provide a means to map both optical-field-controlled electron emission and hot-electron transfer from nanoparticles via chemical transformations produced locally in lithographic materials.