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.

Fabrication of Complex 3D Nanoimprint Mold by Using Acceleration Voltage Electron Beam Lithography

Fabrication of Complex 3D Nanoimprint Mold by Using Acceleration Voltage Electron Beam Lithography

Density multiplication of nanostructures fabricated by ultralow voltage electron beam lithography using PMMA as positive- and negative-tone resist

The authors report a density multiplication process for nanoscale patterns composed of dots and lines using electron beam lithography with low voltage 1 keV exposures and cold development. The density doubling is achieved in a single exposure-development step using polymethylmethacrylate (PMMA) as the resist. PMMA exhibits a dual positive- and negative-tone behavior depending on the electron dose employed in this density multiplication process. Fabricated nanostructures are characterized via scanning electron microscopy and subsequent feature size measurements. After density doubling, the minimum dot diameter of an initially 80 nm pitch array of single pixel dots was measured as approximately 27 nm, and the minimum width in an initially 100 nm pitch array of lines was approximately 21 nm. Methodologies for controlling the dimensions of fabricated structures are discussed. Modeling of the electron beam exposure has been carried out using an original electron beam lithography simulator in order to understand the nominal yields of scission in PMMA required in order to achieve the density multiplication, and the results are discussed.

Developer-free direct patterning of PMMA/ZEP 520A by low voltage electron beam lithography

The authors report an approach that has potential to fabricate dense structures without liquid development. Two kinds of positive tone electron beam resist, 950k PMMA and ZEP 520A (Nippon Zeon), were studied for their properties and behaviors while subjecting them to exposure, thermal development, and reactive ion etching. So far, we have successfully patterned 70 nm half-pitch gratings in both 950k PMMA and ZEP 520A without liquid development.

Nanomachining and clamping point optimization of silicon carbon nitride resonators using low voltage electron beam lithography and cold development

The authors report the nanomachining of sub-20-nm wide doubly clamped silicon carbon nitride resonators using low keV electron beam lithography with polymethyl methacrylate resist and cold development. Methodologies are developed for precisely controlling the resonator widths in the ultranarrow regime of 11–20 nm. Resonators with lengths of 1–20 μm and widths of 16–280 nm are characterized at room temperature in vacuum using piezoelectric actuation and optical interferometry. Clamping and surface losses are identified as the dominant energy loss mechanisms for a range of resonator widths. The resonator clamping points are optimized using an original electron beam lithography simulator. Various alternative clamping point designs are also modeled and fabricated in order to reduce the clamping losses.

Three-dimensional Metal Nano Pattern Transfer on PET using Metal Oxide Layer

There is a strong need for a fine three-dimensional metal patterning technique for fabricating next-generation devices such as patterned media and plasmon photonic and nano-scale electrodes. In addition, flexible and transparent electronic devices on plastic substrates are desired for wearable devices and flexible thin-film displays. Therefore, a technique for patterning metals onto plastic substrates is required. Nanotransfer printing (nTP) has received much attention recently because of its high throughput and high resolution compared to inkjet printing. However, it is difficult to create sub-100 nm metal patterns using nTP because the PDMS stamp is deformed by transfer pressure. Therefore, we have developed a technique for transferring three-dimensional metal patterns onto a polyethylene terephthalate (PET) substrate by nanoimprint lithography using a metal oxide release layer. The three-dimensional nanoimprint mold was fabricated by control of acceleration voltage electron beam lithography (CAV-EBL) with spin-on-glass (SOG). As a result, three-dimensional metal nano patterns were obtained using a metal oxide release layer. Moreover, the metal moth-eye structure, which has a very high aspect ratio, was transferred onto a PET substrate by applying our process to the moth-eye structure on glassy carbon.

Fabrication of three-dimensional nanoimprint mold using inorganic resist in low accelerating voltage electron beam lithography

Three-dimensional (3D) resolution of inorganic resist pattern, which was exposed with control of acceleration voltage electron beam lithography (CAV-EBL) in low accelerating voltage was examined. The system can make features with varying developed-depths. Three-dimensional pattern with a few hundred nanometer linewidth was fabricated with a CAV-EBL. The pattern depths on inorganic resist were gradated with 5 nm depth-resolution per 30 V. By controlling the pattern depth, a seven stairs blade-shaped binary optics mold was fabricated, and then a replica pattern of the mold was made by using UV-NIL.

Metallic air-bridges fabricated by multiple acceleration voltage electron beam lithography

We present a new method of fabricating metallic air-bridge microstructures that is based on a single layer resist and a variation of the electron energy used during the electron beam lithography process. Electrons in the range of 3–30 keV cause radiation-induced reactions in the resists to depths adjustable from fractions of a micrometer up to several micrometers. By varying the energy at which the lithography process is carried out, we obtain three-dimensional profiles in the electron beam resist after exposure and development. Air-bridge structures can then be created by metal evaporation and lift-off.

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.

High sensitivity nanocomposite resists for EUV lithography

A novel nanocomposite photoresist was synthesized for extreme ultraviolet lithography (EUVL) by a radical polymerization process. This resist system exhibited enhanced sensitivity and contrast for EUVL. The potential for EUVL nanofeatures is also examined. The high sensitivity and the desirable contrast in this resist, indicates that it is a promising candidate not only for sub-100 nm resolution EUVL, but also for X-ray lithography and low voltage electron beam lithography.

Line Edge Roughness of Developed Resist with Low-Dose Electron Beam Exposure

Line edge roughness of a developed resist pattern is calculated in the case of low accelerating voltage electron beam lithography. Based on the diffusion equation, the Gaussian distributions of the acid distribution after post-exposure baking in a chemically amplified resist are shown for various conditions, if acid is generated from a point source. It might be considered reasonable to relate the acid broadening to the broadening of the deposited energy, which is originally introduced by the electron beam in the resist. In the present simulation, the energy deposited by the primary electron beam is convoluted with the Gaussian distribution, and the resulting edge roughness variation of the 100-nm-wide line pattern after development is discussed. Because of the convolution, the roughness decreases with increasing diffusion length of the distribution in general, and the dependences of the roughness on the exposure dose, thickness of the resist and the accelerating voltage of the primary beam are discussed.

Charge induced pattern distortion in low energy electron beam lithography

Charge induced pattern distortions in low voltage electron beam lithography in the energy range of 1 to 5 kV were investigated. Pattern distortion on conducting substrates such as silicon was found to be small, while significant pattern placement errors and pattern distortions were observed in the case of electrically insulating substrates caused by charge trapping and deflection of the incident electron beam. The nature and magnitude of pattern distortions were found to be influenced by the incident electron energy, pattern size, electrical conductivity, and secondary electron emission coefficient of the substrate. Theoretical modeling predicts the electron beam deflection to be directly proportional to the trapped surface charge density and inversely proportional to the accelerating voltage.

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.

Low voltage electron beam lithography in PMMA

We have studied low voltage (1–2 kV) electron beam lithography processes in PMMA and compared them to conventional high voltage processing. We looked at the deposited metal after liftoff as well as directly imaging resist profiles by atomic force microscopy. As expected, the proximity effects were greatly reduced. The forward scattering was found to increase at low voltage. The study of developed resist profiles showed that linewidth versus dose has a single Gaussian functional form, proving that forward scattering plays the major role in line broadening. The effective Gaussian linewidth is 60 nm at 1 kV in a 50 nm resist layer. Modeling of the lithographic process showed a significant increase in resolution and process latitude for thinner resists.

Low voltage electron-beam lithography based InGaAs/GaAs quantum dot arrays with 1 meV luminescence linewidths

We have developed InGaAs/GaAs quantum dots with diameters down to about 50 nm on shallow quantum wells using low voltage electron-beam lithography and wet chemical etching. Due to the low energy of the e-beam of 2.5 keV the proximity effect is negligible and arrays of quantum dots with a homogeneous diameter could be fabricated. By using low excitation photoluminescence spectroscopy we observe a clear shift of the dot emission to higher energy due to lateral quantization that amounts to 6 meV in the smallest structures. The linewidth of the luminescence of the dot arrays of about 1.5 meV is almost independent of the dot size, i.e., the inhomogeneous broadening due to the patterning induced lateral size fluctuation is found to be negligible in the present structures.

High resolution electron beam lithography using ZEP-520 and KRS resists at low voltage

ZEP-520 and KRS resist systems have been evaluated as candidates for use in low voltage electron beam lithography. ZEP-520 is a conventional chain scission resist which has a positive tone for over two orders of magnitude in exposure dose. KRS is a chemically amplified resist which can be easily tone reversed with a sensitivity ∼8 μC/cm2 at 1 keV. Both resist systems are shown to have sensitivities ∼1 μC/cm2 for positive tone area exposures to 1 keV electrons. A decrease in contrast in 50 nm thick resist layers is seen when exposure voltage is lowered from 2 to 1 keV, indicating nonuniform energy deposition over the resist thickness. High resolution single pass lines have been transferred into both Si and SiO2 substrates at both low and high voltages in each resist system without using multilayer resist masks. The ZEP-520 and KRS resists are shown to have resolutions of 50 and 60 nm, respectively, at 1 kV, within a factor of 2 of their high voltage resolutions under identical development conditions. A cusp shaped etch profile in Si allows high aspect ratio 20 nm wide trenches to be fabricated using these resists on bulk Si. Low voltage exposures have been used to pattern gratings with periods as small as 75 and 100 nm in ZEP-520 and KRS, respectively. Low voltage exposures on SiO2 show no indications of pattern distortion due to charging or proximity effects.

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.

A special method to create gratings of variable line density by low voltage electron beam lithography

XUV radiation and soft X-rays can be focussed using optical elements that represent extremely smooth and planar substrates with diffracting structures on them. Optical elements of this kind can generally be described as reflection gratings with straight or curved lines of variable line density. Such arbitrary line shapes are easily generated with the help of electron beam lithography. Continuous path control applied in conjunction with data fracturing into BÉZIER curves appears to be particularly suited for this purpose. The method avoids breaking the individual lines down into polygons, which implies an inevitably greater edge roughness. The use of low electron energies allows the individual grid lines to be generated by superpositioning of two path curves without any need to consider the proximity effect. The exposures were carried out on an electron beam lithography system LION-LV11. Measurements performed on BESSY for the first diffraction order showed the resulting optical element to have an efficiency of up to four percent depending on the particular wavelength.

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.

Studies of 1 and 2 keV electron beam lithography using silicon containing P(SI–CMS) resist

We have investigated low voltage electron beam lithography at 1 and 2 keV using P(SI–CMS) resist. With this system, 10:1 aspect ratio, 80-nm-wide polyimide fins, and ≳7:1 aspect ratio, 80-nm-wide Si fins were fabricated using 2 kV exposures. Analysis of our results suggests that observed reductions in the process latitude at 1 kV are not resist specific and can be understood on the basis of electron scattering. Experimental comparisons have been made by exposure of PMMA. In order to describe the results and help guide future development, we have conducted Monte Carlo simulations and experimental studies of the energy deposition in resists by 1 and 2 keV beams.