Surface charging suppression using PEDOT/PSS in the fabrication of three dimensional structures on a quartz substrate

Pattern writing on insulating materials (e.g. quartz) using electron beam lithography (EBL) is a challenging task and it is even more difficult when the pattern is three dimensional (3D). Surface charging trapped on insulating substrates may deflect the electron beam during electron beam pattern writing causing undesired effects.In this work, the surface charging has been suppressed by top coating with water soluble conductive polymer layer using poly (3,4-Etylenedioxythiophene)/poly(styrenesulfonate) (PEDOT/PSS). The 3D masking profiles are created on a negative tone photoresist (Microresist, ma-N2403) using Raith150 EBL tool with variable dose controlled beam exposure. The 3D patterns have been transferred onto the quartz substrate by single step reactive ion etching (RIE) with suitable resist to substrate selectivity.We have demonstrated the fabrication of 3D geometrical shapes such as pyramids, hemispheres, and cones with dimensions down to 300 nm using this technique without any surface charging effects.     

E-beam lithography of computer generated holograms using a fully vectorial 3D beam propagation method

A fully vectorial 3D beam propagation method (BPM) has been applied to obtain a required pattern of computer generated hologram (CGH) with a variable profile of four phase levels. The computer reconstruction of the CGH image having one and two focal spots was performed by application of the fully vectorial 3D BPM method. After transferring the CGH by EBL technique an adequate phase profile was obtained. Inter-level parameter method was developed to obtain the estimated an electron beam dose required for the even topographical patterning. Using this method, an EBL exposure dose determined to achieve the required relief amplitude of 1.29 μm was 43 μC/cm². The manufactured holograms showed that the overall proposed production process, from the 3D BPM computer simulation to e-beam lithography, can be used to obtain good quality product with reasonable time and computational resources.     

Electron beam nanolithography in AZnLOF 2020

We present a lithography process using electron beam lithography with an optical resist AZnLOF 2020 for pattern transfer. High-resolution 100 keV electron beam lithography in 400 nm layers of negative resist AZnLOF 2020 diluted 10:4 with PMGEA is realized. After the electron beam lithography process, the resist is used as a mask for reactive ion etching. We performed the transfer of patterns by RIE etching of the substrate allowing a final resolution of 100 nm. We demonstrate the patterning in an insulating layer, thus simplifying the fabrication process of various multilayer devices; proximity correction has been applied to improve pattern quality and also to obtain lines width according to their spacing. This negative resist is removed by wet etching or dry etching, could allow combining pattern for smallest size down to 100 nm by EBL techniques and for larger sizes by traditional lithography using photomask.     

Structural Transformation of Acrylic Resin upon Controlled Electron‐Beam Exposure Yields Positive and Negative Resists

Zwitter polymers are defined as polymers that undergo transformation from a linear to a crosslinked structure under electron‐beam irradiation. A resist polymer may be either linear or crosslinked, depending on electron‐beam dosage. The structural transformation of acrylic resin make it suitable for applications in positive and negative resists in the semiconductor field. The contrast ratio and threshold dose both increase with increasing resist thickness for both the positive and negative resists, while the positive resist exhibits better contrast than the negative. The intensity of the characteristic Fourier‐transform infrared absorption band at 1612 cm^–1 (vinyl group) is used to explain the phenomena behind these resist transformations. We evaluate the effects of two important processing conditions: the soft baking and post‐exposure baking temperatures. Pattern resolution decreases upon increasing the baking temperature, except for soft baking of the negative resist. The effect of electron dose on the pattern resolution is also discussed in detail for both resists. High electron‐beam exposure does not improve the etching resistance of the resist because of the porous nature of the resist that develops after high‐dosage irradiation.     

Direct stamp fabrication for NIL and hot embossing using HSQ

Commonly stamps or masters for nanoimprinting are made by electron beam lithography (EBL) and subsequent reactive ion etching into silicon. Here we present a single step procedure to prepare stamps suitable for nanoimprinting and hot embossing. The stamps are directly fabricated in HSQ (hydrogen silsequioxane), a negative EBL resist, which has a high lateral resolution and good mechanical properties. We demonstrate successful pattern transfer in both bulk PMMA and PCL by hot embossing with features down to 20 nm. Such pattern transfer is useful for biological applications. Also, we demonstrate that this approach can make stamps suitable for nanoimprint lithography and have achieved features as small as 35 nm. It was found that the stability and strength of the HSQ could be improved by annealing and that the application of a non-stick coating was not necessarily required although it aided the demoulding.     

CO2-Based Dual-Tone Resists for Electron Beam Lithography

The 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 µ C 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.     

Sputtered metal lift-off for grating fabrication on InP based optical devices

Sputtered metal gratings have been realized using lift-off process based on bilayer resist electron beam lithography (EBL). The lithography mask is composed of PMMA (poly(methylméthacrylate)) layer deposited under HSQ (hydrogen silsesquioxane) inorganic resist. EBL is performed in HSQ, whereas PMMA is used to ease final lift-off. We demonstrate the possibility of patterning by lift-off metals with different sputtering yields and deposition conditions. Gratings with period of 200 nm and filling factor of 50% are obtained.     

Anomalous acid diffusion in a triphenylene molecular resist with melamine crosslinker

Next generation lithography will require next generation resists. Molecular resists, based on small non-polymeric molecules, promise improvements in line width roughness and resolution control for high resolution lithographic patterns. However, these materials are generally not sensitive enough for commercial application. We have investigated the application of a common chemical amplification scheme to molecular resists. The triphenylene derivative C5/C0 (symmetrical 2,6,11-trihydroxy-3,7,11-tris(pentyloxy)triphenylene), mixed with the crosslinker hexamethoxymethyl melamine and the photoacid generator triphenylsulfonium triflate shows a substantial sensitivity enhancement, requiring a dose of only 5 μC/cm² compared with the pure triphenylene sensitivity of 6500 μC/cm² at 20 keV. Previous work has indicated that the acid diffusion length of the photoacid generator used here is around 350 nm and that the diffusion length decreases with film thickness. However, in this molecular resist system anomalous levels of acid diffusion were observed, indicating that previous results for polymeric systems may not hold true for these new materials. Initial results indicate that the acid diffusion length in this system may be on the order of microns. Furthermore, there is some evidence that the excessive diffusion is occurring in the surface layers of the resist or at the air: resist interface itself.     

Color stable and low driving voltage white organic light-emitting diodes with low efficiency roll-off achieved by selective hole transport buffer layers

White organic light-emitting diodes (WOLEDs) showing high color stability, low operating voltage, high efficiency and low efficiency roll-off by adopting different hole transport buffer layers which also behaves as electron/exciton blocking layers (EBL) have been developed. The characteristics of WOLEDs based on blue–green and orange phosphors could be easily manipulated by hole transport buffer layer, which tailors charge carrier transportation and energy transfer. Our WOLEDs show low operating voltages, 100 cd/m² at 3.2 V, 1000 cd/m² at 3.7 V and 10000 cd/m² at 4.8 V, respectively, and achieve a current efficiency of 35.0 cd/A, a power efficiency of 29.0 lm/W at a brightness of 1000 cd/m², and a low efficiency roll-off 8.7% calculated from the maximum efficiency value to that of 5000 cd/m².     

Development of metal etch mask by single layer lift-off for silicon nitride photonic crystals

We present a method for fabrication of nanoscale patterns in silicon nitride (SiN) using a hard chrome mask formed by metal liftoff with a negative ebeam resists (maN-2401). This approach enables fabrication of a robust etch mask without the need for exposing large areas of the sample by electron beam lithography. We demonstrate the ability to pattern structures in SiN with feature sizes as small as 50 nm. The fabricated structures exhibit straight sidewalls, excellent etch uniformity, and enable patterning of nanostructures with very high aspect ratios. We use this technique to fabricate two-dimensional photonic crystals in a SiN membrane. The photonic crystals are characterized and shown to have quality factors as high as 1460.     

Electrical properties of Pt interconnects for passive crossbar memory arrays

We report on the fabrication and the electrical characterization of platinum interconnects for novel non-volatile memory technologies. These nanowires present an important and essential contribution to the deep nanometer scaling of alternative architectures beyond CMOS, e.g. nanocrossbar arrays with resistance switching junctions. The nanowires, which have a thickness of 25 nm and a width ranging from 200 nm down to 40 nm, were patterned using electron beam direct writing. They were deposited by UHV electron beam evaporation in combination with a lift-off process.The electrical characteristic is increasingly affected by the contribution of surface effects like scattering at grain boundaries and scattering at the surfaces as the wire dimensions become smaller. With decreasing width of the platinum wire an increasing resistivity was observed, which is consistent with the theories of Fuchs-Sondheimer and Mayadas-Shatzkes. Our studies have shown that the investigated structures possess a high stability concerning the operational current densities up to 4 × 10⁷ A/cm², and an additional annealing step results in an improvement of the electrical wire properties, which is explained by a higher quality of the grain boundaries and side walls.     

Study of X-ray lithographic conditions for SU-8 by Fourier transform infrared spectroscopy

There is growing interest in the use of chemically-amplified resists (CARs) such as SU-8 in the field of microelectromechanical systems (MEMS) research. This is due to its outstanding lithographic performance and its ability for use in the fabrication of stable structures with very high aspect ratio. However, it is important to control the processing conditions for optimum results in the desired application. In this investigation, the thickness (10-25 μm) of SU-8 resist film, due to different spin coating speeds on silicon wafers, was measured using Fourier transform infrared (FT-IR) spectroscopy. The effect of thermal-initiated cross-linking at various temperatures (95-160 °C) for 15 min baking time on the 25 μm SU-8 resist was studied by monitoring the 914 cm⁻¹ absorption peak in the FT-IR spectrum. Results of the experiments showed that the onset of thermal-initiated cross-linking begins at about 120 °C. Furthermore, 25 μm SU-8 resist was optimized for X-ray lithographic applications by studying the cross-linking process of the resist under different conditions of post-exposure bake (PEB) temperatures. The exposure dose of soft X-ray (SXR) irradiation with energies about 1 keV from a dense plasma focus (DPF) device was fixed at 2500 mJ/cm² on the resist surface. Results showed that the optimum processing conditions consisted of an intermediate PEB at 65 °C for 5 min, with the PEB temperature ramped up to 95 °C over 1.5 min and then followed by a final PEB at 95 °C for 5 min. The scanning electron microscopy (SEM) images showed SU-8 test structures successfully imprinted, without affecting the resolution, and with aspect ratios of up to 20:1 on 25 μm SU-8 resist.     

Development of interference lithography for 22 nm node and below

An extreme ultraviolet (EUV) interference lithographic exposure tool was installed at the long undulator beamline in NewSUBARU to evaluate EUV resists for 25 nm node and below. The two-window transmission grating of 40 and 50 nm half pitch (hp) were fabricated with techniques of spattering, electron beam lithography, dry etching and wet etching. hp patterns (20 and 25 nm) of chemically amplified resist (CAR) and non-CAR were successfully replicated using the EUV interference lithographic exposure tool.     

Irradiation damages in electron beam lithography

 The irradiation damages in the electron beam lithography(EBL)to Al-gate MOS capacitors in the ranges of 10—30keV and 10~(-6)—10~(-3)C·cm~(-2) and the effects of annealing on damages at low temperature(<500℃)are given.The research on damages caused by high electron energy(30keV) and ultra-high dosages(10~(-4)—10~(-3) C·cm~(-2))is important and useful to the EBL.The resolution can be improved by high electron energy.Both the EBL with vapor-development and without development are all operated at ultra-high dosages.After irradiations,the concentrations of inter- face states can increase by about one to two orders of magnitude and the flat-band voltages by about a few to more than ten volts.Under constant exposure dosages,the fiat-band voltages are independent of the changes of electron energies in certain energy ranges.Under constant electron energies the concentrations of interface states are independent of the changes of exposure dosages in certain dosage ranges.After annealing,the flat-band voltages can recover the values before the irradiations for energies and dosages in the ranges of 10—30keV and 1×10~(-6)—6×10~(-3)C·cm~(-2) respectively.The interface state concentrations due to the damages of ultra dosages can not be removed completely.     

Efficient electron beam condensing for low-energy microcolumn lithography

Low-energy electron beam lithography has been performed with a microcolumn by adopting a new technique that condenses the electron beam efficiently. To increase the probe current while keeping the kinetic energy of electrons sufficiently low, the negative or positive bias has been applied to the accelerator electrode, which reduces the divergence of the electron beam and hence makes more electrons pass through the microcolumn. With this technique, the probe current more than 1 nA has been achieved, which is large enough for the practical application of microcolumn lithography even when the kinetic energy of electrons is as low as 160 eV. The results of microcolumn lithography by using the condensed electron beam with a low-energy of 160 and 327 eV are also presented.     

Chemical Amplification of a Triphenylene Molecular Electron Beam Resist

Molecular resists, such as triphenylene derivatives, are small carbon rich molecules, and thus give the potential for higher lithographic resolution and etch durability, and lower line width roughness than traditional polymeric compounds. Their main limitation to date has been poor sensitivity. A new triphenylene derivative molecular resist, with pendant epoxy groups to aid chemically amplified crosslinking, was synthesized and characterized. The sensitivity of the negative tone, pure triphenylene derivative when exposed to an electron beam with energy 20 keV was ～ 6 × 10^–4 C cm^–2, which increased substantially to ～ 1.5 × 10^–5 C cm^–2 after chemical amplification (CA) using a cationic photoinitiator. This was further improved, by the addition of a second triphenylene derivative, to ～ 7 × 10^–6 C cm^–2. The chemically amplified resist demonstrated a high etch durability comparable with the novolac resist SAL 601. Patterns with a minimum feature size of ～ 40 nm were realized in the resist with a 30 keV electron beam.     

电子束零宽度线曝光及其应用

为了提高电子束光刻的图形质量及光刻分辨率,从入射束能和束流密度等方面探讨了克服邻近效应影响的途径,在制备亚30 nm结构图形时采用零宽度线曝光的方法。该方法把版图上线条的宽度设为零,因此该线条的光刻尺寸取决于电子束束斑大小、曝光剂量与显影条件。在400 nm厚HSQ抗蚀剂层上通过零宽度线曝光技术制作出了线宽20 nm网状结构的抗蚀剂图形,实验证明采用零宽度线曝光技术可以比较容易地制作出密集线以及高深宽比的抗蚀剂图形。将该技术应用到扫描电镜放大倍率校准标准样品的制备,取得了较好的效果。零宽度线曝光技术是实现电子束直写曝光极限分辨率的有效方法。     

A THz-range planar NDR device utilizing ballistic electron acceleration in GaN

A planar and ultra-short gallium nitride (GaN) diode structure is investigated as a potential Terahertz (THz) range negative differential resistance (NDR) diode. An empirical velocity-field relation, exhibiting a peak electron velocity as high as 7 × 10⁷ cm/s, is employed to characterize the high-field transport in the simulations, accounting for ballistic electron acceleration and velocity reduction due to phonon build up. The resulting device operation is in accumulation-layer transit-time mode and large-signal circuit simulation results are reported along with discussions. Conversion efficiencies up to ∼3.4% at ∼1.5 THz are shown to be possible.     

Fabrication and characterisation of high quality ZnO thin films by reactive electron beam evaporation technique

High quality zinc oxide thin films have been deposited on silicon substrates by reactive e-beam evaporation in an oxygen environment. The effect of the growth temperature and air annealing on the structural, optical and electrical properties has been investigated. X-ray diffraction measurements have shown that ZnO films are highly c-axis-oriented and that the linewidth of the (002) peak is sensitive to the variation of substrate temperature. The optimum growth temperature has been observed at 300 °C. Raman spectroscopy has been found to be an efficient tool to evaluate the residual stress in the as-grown ZnO films from the position of the E₂ (high) mode. On the other hand, the vanishing of the 574 cm⁻¹. Raman feature after annealing has been explained as due to an increase of grain size and the reduction of O-vacancy and Zn interstitial. The SEM images have shown that the surfaces of the electron beam evaporated ZnO became smoother for the growth temperatures higher than 300 °C. The optical transmittance is the highest at 300 °C and has been increased after annealing in air showing an improvement of the optical quality. Finally, the maximum electrical resistivity has been found at 300 °C, which explains its relation with the crystal quality and increased from 5.8×10⁻² Ω cm to reach an approximate value of 10⁹ Ω cm after annealing at 750 °C.     

Improved properties of Al-doped ZnO film by electron beam evaporation technique

High-quality Al-doped ZnO (AZO) thin films have been fabricated by electron beam evaporation technique. The effect of the growth temperature on the optical and electrical properties of the electron-beam (e-beam) evaporated AZO film is investigated. X-ray diffraction measurements have shown that e-beam evaporated films are highly c-axis oriented at appropriate growth temperature. Transmittance measurement showed that the best optical and structural quality of the e-beam evaporated AZO film occurred at 200 °C. The scanning electron microscope images have shown that the surfaces of the e-beam evaporated AZO became smoother for the growth temperature at and above 200 °C. Finally, the maximum electrical resistivity of 2.5×10⁻⁴ Ω cm and optical transmittance of more than 85% has been found at 200 °C growth temperature, which explains its relation with the crystal quality of the film.