High Voltage Electron Beam Lithography Research Articles

High-aspect nano-groove fabrication in thick film resists using 150-kV high acceleration voltage electron beam lithography

High acceleration voltage (150 kV) electron beam lithography is applied to two types of thick film resists with different contrasts (γ). When using 1.6-μm thick high-γ resist, the lowest width discrepancy, 35 nm, was observed at 150 kV in the cross-sectional shape, which was almost vertical. At 50 kV, the discrepancy was more than four times greater than the width at 150 kV. The 50-kV cross-sections were drop shaped. The maximum line widths were similar for both resists, while the top line widths were smaller for the high-γ resist than for the low- γ resist. In addition, Monte Carlo simulation results were consistent with the experimental values, especially at 150 kV.

Miniature Forming Lens for a High-Voltage Electron-Beam Lithography System

This work presents the result of modeling a miniature forming lens for a high-voltage electron-beam lithography system. The diameter of the outer yoke is only 64 mm. This allows the installation of four electron optical systems over a six-inch photomask. For a 50-kV electron beam and 15 mm flange focal distance, the characteristics of the obtained miniature lens provide the absence of yoke saturation and a 25-nm resolution for a 100 × 100 um scanning field. A study of the dependences of the miniature lens’ electron-optical properties on the channel diameter, gap width and yoke length is performed. The obtained results enable selection of the optimal configuration of the forming lens for the development of an electron-beam-lithography system. A four-column composition will allow an increase in the efficiency of the lithography process by up to four times.

A new nanocomposite resist for low and high voltage electron beam lithography

A novel nanocomposite photoresist was synthesized and characterized for use in both low and high voltage electron beam lithography. This resist system is shown to display the ideal combination of both enhanced etch resistance and enhanced sensitivity required to satisfy both low and high voltage patterning applications. Resist sensitivity was enhanced by the direct incorporation of a photoacid generating monomer into the resist polymer backbone while the etch resistance of the material was improved by copolymerization with a POSS containing monomer.

Optimum energy for high resolution low voltage electron beam lithography — Monte Carlo simulations and experiments

The electron backscattering coefficients of thin tri-level-process layer systems in the energy range from 2.5 to 40.0 keV have been calculated by Monte Carlo simulations. The results show a significant variation of the backscattering with the primary electron energy and the atomic number of the materials used in the layer systems. A local backscattering minimum in the region of 4–5 keV has been found for the investigated multilayer systems. Its stability to variations of the layers' parameters has been investigated. First high resolution experiments at 5.0 keV primary electron energy were carried out to determine the minimal achievable grating period with line-to-space ratio of 1:1. Using ZEP-7000 and hexaacetate p-methylcalix[6]arene resist gratings with 60 and 50 nm minimum period, respectively, were manufactured.

High voltage electron beam nanolithography on WO3

High voltage electron beam lithography experiments on thin films of tungsten trioxide are reported. For linear doses around 10 mC/cm at 200 kV, a negative resist behavior is observed after development in a NaOH solution. Features with dimensions as low as 15 nm with aspect ratios up to 10, exhibiting weak size fluctuations and vertical sidewalls are routinely achieved. Proximity effects are found to be negligible allowing the fabrication of structures with less than 10 nm separation. The exposure mechanism is investigated by transmission electron microscopy and diffraction. A crystallization process induced by the electron bombardment is clearly evidenced. A model based on the amorphous to crystalline phase transition induced by electron irradiation is proposed to account for the experimental observations.

Fabrication of ultrasmall magnets by electroplating

We use high voltage electron beam lithography followed by electroplating to define small metal features on semiconductor substrates. These have been used to form high resolution etch masks, dense nanomagnet arrays, and highly anisotropic metal nanostructures. To reproducibly obtain uniform arrays of such structures, we have developed an end-point detection technique, which is based on in situ observation of the electrodeposition process.

Patterning of an electron beam resist with a scanning tunnelling microscope operating in air

A scanning tunnelling microscope operating in air is employed to expose a 50 nm thick electron beam sensitive resist with low energetic electrons. We demonstrate that it is possible to expose the resist under ambient conditions with voltages of ~50 V without observable modification of the resist surface after exposure prior to development. A resolution of 150 nm has been achieved. The dose for complete exposure has been determined as 10 mC cm −2. This is about 1000 times higher than the value for conventional high-voltage electron beam lithography.

Strong lateral quantization effects in the luminescence of InGaAs/InP quantum wires

We have fabricated In 0.53Ga 0.47As/InP quantum wires with lateral widths down to 8 nm by high voltage electron beam lithography and deep wet chemical etching. The wires were studied by photoluminescence spectroscopy at room temperature and at 2 K. We observe only a weak decrease of the luminescence yield with decreasing wire width, which indicates that no significant damage has been induced at the sidewalls of the wires during the fabrication process. For wires with widths smaller than about 60 nm an increasing blue shift of the photoluminescence energy up to about 75 meV (for 8 nm wide wires) is observed indicating a strong lateral quantization. The photoluminescence emission of the wires is strongly polarized parallel to the wire orientation with increasing polarization degree for decreasing wire width.

Room temperature study of strong lateral quantization effects in InGaAs/InP quantum wires

We have fabricated In0.53Ga0.47As/InP quantum wires with lateral widths down to 8 nm by high voltage electron beam lithography and deep wet chemical etching. The wires were studied by photoluminescence spectroscopy at room temperature. Down to the smallest wire widths a clear photoluminescence signal is observed. The decrease of the luminescence yield with decreasing wire width indicates that no significant damage has been induced at the sidewalls of the wires during the fabrication process. For wires with widths below about 60 nm a blue shift of the photoluminescence energy is observed, which reaches up to 73 meV for 8 nm wide wires. The experimentally observed width variation of the lateral quantization can be modeled quantitatively by using the measured width of the wires and standard band parameters.

Fabrication of sub-20 nm trenches in silicon nitride using CHF3/O2 reactive ion etching and oblique metallization

A direct process for fabricating nanometer size trenches in Si3N4 using high voltage electron beam lithography and CHF3/O2 reactive ion etching has been developed and characterized. The process can be used on both bulk and thin membrane substrates and has demonstrated a feature resolution of better than 20 nm. An extension of this process allows 15 nm wide slots to be fabricated in a metal film without performing any metal etching.

100 kV thermal field emission electron beam lithography tool for high-resolution x-ray mask patterning

High-voltage (≥50 kV) electron beam lithography (EBL) is the preferred technique for fabrication of additive-process x-ray masks, because the high-voltage minimizes scattering in the resist and membrane, resulting in better resolution, straighter sidewalls, and reduced proximity effect. We have designed and built a new 100 kV column for a vector scan EBL machine for the purpose of writing high-resolution, high-precision x-ray masks in order to explore the technological and fundamental limits of x-ray lithography. The column features a 100 kV thermal field emission gun with an electrostatic condenser lens, conjugate blanking, and a liquid-cooled magnetic final lens with high-precision double magnetic deflection. The two-lens optics provides a beam diameter of 30 nm at a current of 5 nA, sufficient to expose moderately sensitive resists at pixel rates approaching the maximum deflection speed of 10 MHz. Results obtained include proximity corrected, complex patterns in thin resist with feature sizes down to 50 nm. Comparisons of proximity effects, exposure parameters, and actual resist profiles, show that 100 kV is clearly superior to 50 kV and even 75 kV for feature sizes below 0.25 μm in thick (0.75 μm) resist. Excellent linewidth control has been obtained on plated gold x-ray masks with feature sizes as small as 75 nm. Problems of patterning nanometer features with aspect ratios as high as 10:1, which include forward scattering, development effects, and plating effects, are discussed.

T-gate, Γ-gate, and air-bridge fabrication for monolithic microwave integrated circuits by mixed ion-beam, high-voltage electron-beam, and optical lithography

Mixed lithography combining a 140 keV Be++ focused ion beam and a 60 keV electron beam has been applied to fabricate T-section and Γ-section structures for gate electrodes in GaAs metal–semiconductor field effect transistors for monolithic microwave integrated circuits. The gates are formed by lift-off metallization, following successive electron and ion beam exposures of a single resist layer and a single stage of development. Gates with lengths down to 170 nm have been fabricated. Gate resistance parameters given by reverse modeling with data from microwave measurements are consistent with direct measurements on test structures. Forward modeling shows the benefits of the Γ-gate structure for well-balanced device design. High-voltage lithography has been applied also to a novel procedure for generating air-bridges for monolithic microwave integrated circuits.

High voltage electron beam lithography of the resolution limits of SAL 601 negative resist

We have investigated the suitability of SAL 601 for high voltage electron beam lithography. For exposure energies in excess of 100 keV resist lines with widths below 50 nm can be routinely defined. The resist masks have been used for the fabrication of quantum wires by wet etching.

Proximity effect in high voltage electron beam lithography on Ti/Pt/Au metallization

The point exposure distribution function (PEDF) for electron beam lithography on Ti/Pt/Au has been measured for 50 keV and 100 keV electrons. The PEDF's show strong non-Gaussian behaviour. The range of the backscattered electrons does not change whereas the backscatter coefficient is significantly reduced when the electron energy is increased from 50 keV to 100 keV. The data is used in calculations of the dose distribution for a typical Ti/Pt/Au metallization pattern of a microwave bipolar transistor. The results combined with the measured contrast function, provide information over the process parameters needed for well-defined resist patterns and confirm the narrow parameter range observed in preliminary experimental work.

Fabrication of sub-20 nm structures in silicon nitride using CHF 3/O 2 rie

A simplified process for fabricating nanometre-scale trenches in Si 3N 4 using high voltage electron beam lithography and CHF 3/O 2 RIE has been developed. The process can be used on both bulk and thin substrates and has demonstrated a resolution of better than 20nm.

Optical Properties of InGaAs/InP Quantum Wires Defined by High Voltage Electron Beam Lithography at 200 kV

ABSTRACTWe have fabricated and analyzed high quality InGaAs/InP quantum wires by electron beam lithography and wet chemical etching. In order to optimize the shape of the wet-etched wires different wire orientations were investigated. As results of the lithography process we obtain wire masks with widths down to 15 nm and etched wires with widths of the InGaAs layer of 18 nm.The wires were studied optically by means of photoluminescence spectroscopy. In contrast to dry etched wire structures the wet chemically etched wires show strong optical emission even for geometrical widths less than 25 nm. The weak decrease of the quantum efficiency with decreasing wire width indicates that there are no dead layers at the side walls of the wires, which is in contrast to previous studies on dry-etched structures. The photoluminescence energy of the InGaAs/InP wires is independent of the wire dimension down to widths of 50 nm. This indicates that a steep lateral potential in our structures is obtained due to the confinement by the semiconductor/vacuum transition at the etched surfaces. For wires with smaller widths an increasing blue shift of photoluminescence energy up to more than 30 meV is observed.

Proximity effect correction for high-voltage electron beam lithography

A new proximity effect correction method using an approximate dose correction formula has been developed. The method is especially effective for a high acceleration voltage. In the case of an acceleration voltage of 40 kV, the correction error for the energy deposition in the resist was estimated analytically to be less than ±7%. The deviations of the line and space pattern dimensions were also evaluated experimentally to be less than ±0.03 μm. The calculation time for correcting a 4-Mbit dynamic random access memory (DRAM) was only 0.7 h using a large-scale computer of 15 MIPS (mega instructions per second). Even if the LSI patterns have no hierarchical structure, in an application specific integrated circuit (ASIC) pattern the calculation time would be 1 h or so. A 4-Mbit DRAM pattern can be written using the electron beam direct-writing system EX-7 with sufficient accuracy. These results suggest that the method is practical.

Nanowriter: A new high-voltage electron beam lithography system for nanometer-scale fabrication

A new high-voltage electron beam lithography system, Nanowriter, has been designed and constructed to give a large scan field size without compromising the ability to write nanometer-scale structures. Nanowriter comprises a 100-kV LaB6 electron gun with a zoom condenser arrangement consisting of two condenser lenses which are used to provide a system demagnification of up to 10 000 times. To combine a very small on-axis spot diameter of 4 nm and a large scanned field of 250×250 μm without dynamic correction, a newly developed swinging objective immersion lens (SOIL) concept based on variable axis immersion lens (VAIL) and swinging objective lens (SOL) has been utilized. The SOIL used here has four advantages: first, it is possible to reduce the working distance, thereby decreasing spherical aberration; second, the scanned field is increased without significantly increasing deflection aberrations; third, it permits easy alignment of the column, and fourth, an inherent attribute of the SOIL is the shielding from extraneous magnetic fields given to the target. The substrate resides on an in-lens stage which is supported on the boreless lower pole piece of the SOIL. The system has been used to delineate nanometer-scale patterns in poly(methylmethacrylate) (PMMA) resist. Lines of 15-nm width have been written into 50-nm-thick PMMA on thick Si substrates. These linewidths were almost unchanged over the whole scanned area of 250×250 μm.

Ultranarrow conducting channels defined in GaAs-AlGaAs by low-energy ion damage

We have laterally patterned the narrowest conducting wires of two-dimensional electron gas (2DEG) material reported to date. The depletion induced by low-energy ion etching of GaAs-AlGaAs 2DEG structures was used to define narrow conducting channels. We employed high voltage electron beam lithography to create a range of channel geometries with widths as small as 75 nm. Using ion beam assisted etching by Cl2 gas and Ar ions with energies as low as 150 eV, conducting channels were defined by etching only through the thin GaAs cap layer. This slight etching is sufficient to entirely deplete the underlying material without necessitating exposure of the sidewalls that results in long lateral depletion lengths. At 4.2 K, without illumination, our narrowest wires retain a carrier density and mobility at least as high as that of the bulk 2DEG and exhibit quantized Hall effects. Aharonov–Bohm oscillations are seen in rings defined by this controlled etch-damage patterning. This patterning technique holds promise for creating one-dimensional conducting wires of even smaller sizes.

Fabrication of GaAs heterojunction ring structures

Micron-sized loops of two-dimensional electron gas have been made on GaAs-AlGaAs heterostructures using high-voltage electron beam lithography, for the investigation of the physics of low-dimensional structures, such as quantum interference (weak localisation), the Aharonov-Bohm (AB) effect and the Quantum Hall effect. The fabrication procedure has been designed to minimise the effects of electron irradiation. Marked AB oscillations have been observed at low temperatures (T<100mK).