Low Voltage Electron Beam Lithography Research Articles

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.

Effects of fast secondary electrons to low-voltage electron beam lithography

In 1981 A. N. Broers suggested that the spatial limit of direct writing electron beam lithography (DWEBL) would be limited to ~10 nm by the laterally scattered fast secondary electrons (FSE) even in atomically thin resist. One possible solution to this restriction would be to use low- or ultralow-energy electrons. Experiments and simulations have been carried out to quantify the contribution of FSE to the energy deposition that results in exposure of the resist over high-beam energies. To examine the effects of FSE on low-voltage operations, studies of electron-beam lithography (EBL) in the low- to ultralow-energy range, employing commonly used resists such as PMMA, were performed, and the results were compared to those from conventional high-voltage processing. DWEBL was performed in a Schottky field emission gun scanning electron microscope (SEM), used in cathode-lens mode for ultralow-voltage operation. The exposure characteristics and sensitivity of the system at these energies have been investigated using Monte Carlo simulation methods. Saturation doses were calculated at low energies, which would give a useful condition to target for routine exposure because it ensures the critical dimensions will not be affected by any random changes in beam intensity.

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.

Bioactive Templates Fabricated by Low-Energy Electron Beam Lithography of Self-Assembled Monolayers

Adhesive templates for biomolecule patterning were fabricated on silicon and gold by low-voltage (1 kV) electron beam lithography of an inert self-assembled monolayer, followed by backfilling the exposed regions with an amine-terminated monolayer. Amine-terminated monolayers selectively attached either the desired materials or linker molecules that subsequently bound other materials including antibodies. Lines (300 nm wide) of 20-nm polystyrene beads were formed on gold by exposing a mercaptohexadecanoic acid (MHDA) monolayer, then backfilling with cysteamine, and selectively attaching aldehyde-coated beads to the amines. Attachment density was found to vary sharply around a critical dose, making the technique useful for patterns such as gradients which require varying density. An optimal dose of 200 μC/cm2 was found for attaching fluorescent polystyrene spheres to MHDA−cysteamine templates. A cycling process was developed for aligning patterns of two or more kinds of polystyrene spheres. Biotin was tethe...

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.

Nanometer-Scale Patterning of Polystyrene Resists in Low-Voltage Electron Beam Lithography

We studied nanometer-scale patterning using a polystyrene negative resist in electron beam lithography. We found that the use of a low-molecular-weight polystyrene enables 10-nm-level patterning at low-acceleration voltage. We also found that the spot dose of such ultrasmall patterns formed at a 5 kV acceleration voltage was one-tenth of that formed at 50 kV. Low-voltage electron beam lithography is a suitable technique for organic resist nanopatterning. The Charlesby theory can still be applied to nanodot formation, and we can therefore estimate the dot sensitivity for various polystyrene molecular weights. We suppose that an exposure model is based on polymer aggregation to explain the formation of a 10-nm-level pattern with a height of 40 nm can be formed by using a small molecule, not a large one.

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.

Exciton interaction effects in the emission spectra of single free-standing InGaAs/GaAs quantum dots

We report on highly spatially resolved photoluminescence studies on individual free-standing InGaAs/GaAs quantum dots with diameters between 100 and 40 nm. Single dots with spacings of 20 μm between adjacent structures have been developed by low-voltage electron-beam lithography and wet chemical etching. Photoluminescence spectra taken at low excitation densities systematically shift to higher energies with decreasing dot diameter. With increasing excitation power, the emission includes additional peaks at higher energies due to transitions between the second and third quantized levels. Simultaneously, a renormalization of the transition energies by up to 2 meV and a significant broadening of the spectra are observed. The energy renormalization and spectral broadening are discussed on the basis of Coulomb interaction between excitons inside the dot and carrier pairs in the surrounding GaAs barriers.

Low-voltage electron-beam lithography with scanning tunneling microscopy in air: A new method for producing structures with high aspect ratios

A new lithography method for patterning an electron-beam resist employed in microelectronic technology with low energetic electrons from a scanning tunneling microscopy tip is presented. The method utilizes a thin electron-sensitive, chemically amplified top resist (positive mode) and amorphous diamondlike carbon as conductive bottom resist which allows one (in contrast to other scanning probe microscopy techniques) to achieve aspect ratios larger than five. The exposure response curve of the employed top resist is presented and compared with high-energy e-beam results. It is shown that the exposure with low-energy electrons leads to a direct cleavage of the functional group of the resist polymer responsible for development. Furthermore, the achievable writing speed for scanning tunneling microscopy exposure is estimated. The successful application of the scanning-tunneling-microscopy-based lithography is demonstrated especially with respect to the high aspect ratio on various substrates (Al, Si, SiO2, and poly Si).

Photoluminescence study of deep etched InGaAs/GaAs quantum wires and dots defined by low-voltage electron beam lithography

Combining low-voltage electron beam lithography (EBL) and wet chemical etching, arrays of deep etched InGaAs/GaAs quantum wires with widths down to 15 nm and dots with minimum diameters of 27 nm have been fabricated. The application of low-voltage EBL strongly reduces the proximity effect during pattern exposure and allows the formation of very homogeneous nanostructure arrays. Low-excitation photoluminescence (PL) spectroscopy of both wires and dots reveals a structure size dependent blue shift of the emission lines up to 14 meV. This energy shift is caused by lateral confinement and shows a clear dependence on the structure dimensionality. The quantization is calculated with a simple model using only standard InGaAs/GaAs material parameters and the geometrical structure widths measured with a scanning electron microscope (SEM).

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.

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.

InGaAs/GaAs quantum wires and dots defined by low-voltage electron-beam lithography

Low-voltage electron-beam lithography (EBL) of energies of 2.3 keV is a powerful tool for efficient exposure of poly(methylmethacrylate) resist patterned substrate, with negligible proximity effects. InGaAs/GaAs quantum wires and dots have been fabricated with lateral wire widths down to 15 nm, and dot diameters down to 37 nm, respectively. The nanostructures were defined by combining high-resolution low-voltage EBL and deep wet chemical etching. Quantum wires and dots show a dimensionality dependent photoluminescence energy blue shift of up to 12 meV, caused by lateral quantization.

Resists and processes for 1 kV electron beam microcolumn lithography

Low-voltage electron beam lithography has been considered for some time for high-resolution lithography because of reduced proximity effects, increased resist sensitivities, and reduced substrate damage. An accelerating voltage of 1 kV is being considered for the operation of microcolumns that are being developed for use in an innovative, high-speed, high-resolution electron beam lithography system. The development of resists and processes for use at 1 kV then becomes one of the essential components in this developing technology. Since 1 keV electrons have a penetration depth of ∼60 nm in organic polymers, the resists used for electron beam exposures must be correspondingly thin. This makes the development of processes for pattern transfer a challenge. Here, we report on three resist and processing schemes for use at 1 kV which can be used to produce 50–100 nm wide structures. Structures as small as 50 nm wide with 3 to 1 aspect ratios, and trenches 300 nm deep and less than 100 nm wide have been successfully transferred into Si substrates. Despite the use of very thin resists, the transferred patterns do not suffer from noticeable defects.

Low-voltage electron beam lithography on GaAs substrates for quantum wire fabrication

We have studied the benefits of low-voltage electron beam lithography (EBL) in polymethyl methacrylate (PMMA) positive electron beam resist for the fabrication of quantum wire etch masks on GaAs substrates. In order to determine the reduction of electron scattering effects for low-voltage EBL, we have compared point exposures in 100-nm-thick PMMA for energies between 2.3 and 15 keV. The exposure distribution functions were fitted by combinations of Gaussians and exponentials, showing significant energy-dependent deviations from the widely used double-Gaussian function. The results indicate that on GaAs substrates, backscattering at 2.3 keV is reduced by about two orders of magnitude compared to 15 keV exposures. InGaAs/GaAs quantum wires have been fabricated using low-voltage EBL, followed by an aluminum-liftoff step and wet-chemical etching. In sharp contrast to wires delineated at 25 keV, the resulting structures show no width variations at the field edges caused by proximity effects. The photoluminescence spectra show a wire width dependent blue shift up to 8 meV for 18-nm-wide structures, caused by lateral quantization.