Low-energy Electron-beam Lithography Research Articles

Band-to-Band Tunneling-Dominated Thermo-Enhanced Field Electron Emission from p-Si/ZnO Nanoemitters.

Thermo-enhancement is an effective way to achieve high performance field electron emitters, and enables the individually tuning on the emission current by temperature and the electron energy by voltage. The field emission current from metal or n-doped semiconductor emitter at a relatively lower temperature (i.e., < 1000 K) is less temperature sensitive due to the weak dependence of free electron density on temperature, while that from p-doped semiconductor emitter is restricted by its limited free electron density. Here, we developed full array of uniform individual p-Si/ZnO nanoemitters and demonstrated the strong thermo-enhanced field emission. The mechanism of forming uniform nanoemitters with well Si/ZnO mechanical joint in the nanotemplates was elucidated. No current saturation was observed in the thermo-enhanced field emission measurements. The emission current density showed about ten-time enhancement (from 1.31 to 12.11 mA/cm2 at 60.6 MV/m) by increasing the temperature from 323 to 623 K. The distinctive performance did not agree with the interband excitation mechanism but well-fit to the band-to-band tunneling model. The strong thermo-enhancement was proposed to be benefit from the increase of band-to-band tunneling probability at the surface portion of the p-Si/ZnO nanojunction. This work provides promising cathode for portable X-ray tubes/panel, ionization vacuum gauges and low energy electron beam lithography, in where electron-dose control at a fixed energy is needed.

In Situ Patterning of Ultrasharp Dopant Profiles in Silicon.

We develop a method for patterning a buried two-dimensional electron gas (2DEG) in silicon using low kinetic energy electron stimulated desorption (LEESD) of a monohydride resist mask. A buried 2DEG forms as a result of placing a dense and narrow profile of phosphorus dopants beneath the silicon surface; a so-called δ-layer. Such 2D dopant profiles have previously been studied theoretically, and by angle-resolved photoemission spectroscopy, and have been shown to host a 2DEG with properties desirable for atomic-scale devices and quantum computation applications. Here we outline a patterning method based on low kinetic energy electron beam lithography, combined with in situ characterization, and demonstrate the formation of patterned features with dopant concentrations sufficient to create localized 2DEG states.

Water soluble and metal-containing electron beam resist poly(sodium 4-styrenesulfonate)

Popular electron beam resists such as PMMA, ZEP and HSQ all use solvent or base solutions for processing, which may attack the sub-layers or substrate that are made out of organic semiconducting materials. In this study we show that water soluble poly(sodium 4-styrenesulfonate), or sodium PSS, can be used as a negative electron beam resist developed in water. Moreover, since PSS contains metal sodium, its dry etching resistance is much higher than PMMA. It is notable that sodium PSS’s sensitivity and contrast is still far inferior to organic resists such as PMMA, thus it is not suitable for patterning dense and high-resolution structures. Nevertheless, feature size down to 40 nm was achieved for sparse patterns. Lastly, using very low energy (here 2 keV) electron beam lithography and liftoff process using water only, patterning of metal layer on an organic conductive material P3HT was achieved. The metallization of an organic conducting material may find applications in organic semiconductor devices such as OLED.

Area-Selective ALD of TiO2 Nanolines with Electron-Beam Lithography

We demonstrate a bottom-up approach to fabricate nanoline structures using self-assembled monolayer (SAM) modified substrates to selectively prevent nucleation during atomic layer deposition (ALD). Low-energy (≤5 kV) electron-beam lithography (EBL) was used to modify the hydrophobic functional groups (−CH3) of octadecyltrichlorosilane (OTS) SAM to hydrophilic species (e.g., −COOH), which allows chemisorption of the titanium isopropoxide (TTIP) and water to initiate titanium oxide (TiO2) nucleation. TiO2 thin films were selectively deposited on the OTS molecules which were properly functionalized or patterned. We systematically investigate the effects of e-beam dose and accelerating voltage on selective TiO2 deposition with nanoline patterns. The results indicate that the former parameter determines the resolution of individual line width, while the latter one is attributed to the minimum pitch dimension of dense line patterns achievable. Using the optimal e-beam parameters, i.e., accelerating voltages of 1–2 kV and a line dose of 10 nC/cm, TiO2 nanolines with a maximum resolution of 30 nm and a minimum pitch of 50 nm were achieved. This study offers a new approach to fabricate close-packed nanopatterns for IC devices without any challenging etching processes.

High resolution HSQ nanopillar arrays with low energy electron beam lithography

Electron beam lithography (EBL) is commonly used for the fabrication of nanostructures by top-down approach with precise control of size, shape, aspect ratio, and location. In this article, we demonstrate the realization of high aspect ratio nanopillars (7.5) with 20nm diameter in 150nm Hydrogen SilsesQuioxane (HSQ) thickness based on 20keV energy exposure. A detailed study of design strategies has been conducted in order to correlate the design of the nanopillars and their practical realization. We describe an original way of scanning for HSQ nanopillar arrays fabrication with almost perfect anisotropic sidewalls (98.5%) and good circularity shape (1.62nm-1@s) without any residual resist remaining around the nanostructures.

New method of optimizing writing parameters in electron beam lithography systems for throughput improvement considering patterning fidelity constraints

Low-energy electron beam lithography is one of the promising next-generation lithography technology solutions for the 21-nm half-pitch node and beyond because of fewer proximity effects, higher resist sensitivity, and less substrate damage compared with high-energy electron beam lithography. To achieve high-throughput manufacturing, low-energy electron beam lithography systems with writing parameters of larger beam size, larger grid size, and lower dosage are preferred. However, electron shot noise can significantly increase critical dimension deviation and line edge roughness. Its influence on patterning prediction accuracy becomes nonnegligible. To effectively maximize throughput while meeting patterning fidelity requirements according to the International Technology Roadmap for Semiconductors, a new method is proposed in this work that utilizes a new patterning prediction algorithm to rigorously characterize the patterning variability caused by the shot noise and a mathematical optimization algorithm to determine optimal writing parameters. The new patterning prediction algorithm can achieve a proper trade-off between computational effort and patterning prediction accuracy. Effectiveness of the new method is demonstrated on a static random-access memory circuit. The corresponding electrical performance is analyzed by using a gate-slicing technique and publicly available transistor models. Numerical results show that a significant improvement in the static noise margin can be achieved.

Lithography-patterning-fidelity-aware electron-optical system design optimization

Low-energy electron beam lithography is a promising patterning solution for the 21 nm half-pitch node and beyond due to its high resolution, low substrate damage, and increased resist sensitivities. To ensure a successful electron-optical system (EOS) design, many factors such as focusing properties (FPs) and patterning fidelity (PF) have to be considered. In traditional EOS optimization flow, FPs are typical performance indices selected when optimizing the EOS design parameters. In each numerical iteration, the EOS FP simulation results are compared with specified performance index values. The differences are reduced by adjusting the EOS design parameters until convergence. However, the performance indices related to FPs may have no direct relation to lithography PF, which is judged by the quality of the developed resist patterns. A new EOS design methodology which directly incorporates lithography PF metrics into the optimization flow is proposed. The EOS design parameters are first optimized while meeting the geometric constraints by using the traditional design flow to obtain acceptable FPs. In order to ensure lithography PF, writing patterns are selected and writing parameters are optimized. Then, constraints and cost functions related to PF are selected to further optimize the EOS design parameters to obtain acceptable PF. In each numerical iteration, the simulated lithography patterning results are compared against specified PF metric values. Their differences are reduced by adjusting the EOS design parameters until all constraints are met and PF cost functions are converged. The proposed method is applied to an EOS structure design for a 5 keV electron beam lithography system which includes a single-gate source and a focusing lens. Initial values of EOS design parameters and geometric constraints are selected based on previous studies. A drawn layout for a 22 nm isolated line pattern is used for verifying the lithography PF specifications based on the International Technology Roadmap of Semiconductors. The developed resist pattern after applying the proposed method clearly indicates that the PF is significantly improved from the value of corresponding critical dimension (CD) and the value of gate CD control.

Interdependence of optimum exposure dose regimes and the kinetics of resist dissolution for electron beam nanolithography of polymethylmethacrylate

The authors report a systematic experimental study of dense nanostructures in polymethylmethacrylate (PMMA) created by low-energy electron beam lithography (EBL) with varying duration and temperature of the resist dissolution. They observe that decreasing the development temperature not only yields the widest favorable exposure dose regimes but also requires highest exposure doses to fabricate dense nanopatterns. They interpret the observed interdependence of the exposure doses and the development temperatures in terms of a simple kinetic model describing the diffusion mobility of fragments in exposed PMMA during dissolution and discuss the corresponding molecular mechanisms that determine the resolution and sensitivity of EBL nanofabrication.

Amino-propyl-triethoxy-silane on aluminum fiducial grids for spatial-phase-locked electron-beam lithography

A new fiducial grid suitable for low-energy (1–5keV) spatial-phase locked electron-beam lithography (SPLEBL) has been developed based on an aluminum layer with an organosilane grid pattern. The grids (400nm period) were made of two to three layers of 3-amino-propyl-triethoxy-silane (APTES) that was microcontact printed onto Al coated polymethyl methacrylate (PMMA). Signal-to-noise ratios (SNRs) were measured at beam energies ranging from 1to5keV at a dose of 20μC∕cm2 and compared to the SNRs of octadecanethiol (ODT) based fiducial grids on Au, Ag, and Cu. Although fiducial grids made of ODT on Au provided excellent SNR, Au is not suitable because of poor penetration and strong scattering of the primary electrons. ODT based grids on lower atomic number metal such as Ag or Cu yielded inadequate SNRs. In contrast, APTES-based grids on Al provide SNRs approaching ODT on Au with greatly reduced electron scattering. The authors successfully developed a grid removal process that allows patterns to be resolved in the underlying PMMA. These results represent a key step in the development of low-energy SPLEBL.

Photo-mask fabrication by low-energy microcolumn lithography

We have designed and fabricated a low-energy electron-beam lithography system based on a single column module (SCM) microcolumn. From the observed characteristics of the polymethyl methacrylate (PMMA) resist, the optimum conditions for the low-energy e-beam lithography have been determined. Fine line patterns on PMMA with line width less than 60 nm were obtained under optimized lithographic conditions. For the first time an aluminum photo-mask for optical lithography was created utilizing microcolumn lithography. Our results show that low-energy lithography systems have the potential to be used in high quality photo-mask fabrication processes.

Self-assembled monolayer fiducial grids for spatial-phase-locked electron-beam lithography

The authors characterize a new fiducial grid based on a self-assembled monolayer (SAM) that is well suited to low-energy (⩽10keV) spatial-phase locked electron-beam lithography (SPLEBL). SAMs significantly alter the secondary electron yield of the metal films on which they are formed. In addition, SAMs are not expected to strongly scatter the primary beam, even at low energies, because they are less than 2-nm-thick and are composed of low atomic number elements. In this work the authors evaluate the signal-to-noise ratio (SNR) of SAM grids on gold and copper coated electron-beam resist. 400nm period octadecanethiol fiducial grids were microcontact printed onto the gold and copper metal layers using polydimethylsiloxane stamps. Gold serves as a model system and provides excellent SNR; however, its strong forward scattering makes it impractical in many applications. Copper offers reduced forward scattering but exhibits inverted secondary electron contrast and greatly reduced SNRs. In all cases, SNR decreases with increasing beam energy as overall secondary electron yield decreases. These results suggest that SAM fiducial grids are promising for low-energy SPLEBL; however, further optimization of the interlayer and SAM composition is warranted.

Feasibility study of TFT-LCD array tester using low voltage micro-columns

Since Chang et al. reported the pioneering concept of micro-column, researches on the micro-column have been devoted to its application to low energy electron beam lithography with high throughput capabilities using arrayed micro-column structure [E. Kratschmer, H.S. Kim, M.G.R. Thomson, K.Y. Lee, S.A. Rishton, M.L. Yu, S. Zolgharnain, B.W. Hussey, T.H.P. Chang, J. Vac. Sci. Technol. B 14(6) (1996) 3792; T.H.P. Chang, M.G.R. Thomson, E. Kratschmer, H.S. Kim, M.L. Yu, K.Y. Lee, S.A. Rishton, B.W. Hussey, S. Zolgharnian, J. Vac. Sci. Technol. B 18(6) (1996) 3774; L.P. Muray, J.P. Spallas, C. Stebler, K. Lee, M. Mankos, Y. Hsu, M. Gmur, T.H.H. Chang, J. Vac. Sci. Technol. B 18(6) (2000) 3099; H.S. Kim, D.W. Kim, S. Ahn, Y.C. Kim, J. Cho, S.K. Choi, D.Y. Kim, Microelectron. Eng. 55 (2005) 78–79]. Also, developing inspection systems using micro-columns can be a promising technology for the production of next generation devices with narrow line-width since the low energy (<1keV) beam can minimize the damage of devices. In this experiment, we have assembled a TFT-LCD array tester with a micro-column and tested its performance as a feasibility study.

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.

저 에너지 초소형 전자칼럼 리소그래피를 이용한 SiO2박막의 Pattern 제작에 관한 연구

반도체의 고 집적회로를 형성하기 위하여 주로 이용하고 있는 광 리소그래피 기술을 대신하여 사용할 수 있는 차세대 리소그래피 기술로 전자빔 리소그래피 기술에 대한 연구가 진행되고 있다. 본 연구에서는 초소형 전자칼럼을 이용하여 전자빔 에너지와 조사농도에 따른 pattern 두께의 의존성을 조사하였으며 두께가 100nm인 <TEX>$SiO_2$</TEX> 박막의 patterning을 통하여 <TEX>$SiO_2$</TEX> 박막에 대한 저 에너지 전자빔 리소그래피 공정의 가능성을 입증하였다. Electron beam lithography has been studied as a next-generation lithography technology instead of photo lithography for ULSI semiconductor devices. In this work, we have made a low-energy electron beam lithography system based on the microcolumn and investigated the dependence of the pattern thickness on the energies and dose concentration of the electron beam. We have also demonstrated the potential of low-energy lithography by achieving 100 nm-<TEX>$SiO_2$</TEX> thin film patterning.

Nanoscale resist morphologies of dense gratings using electron-beam lithography

The authors report a systematic investigation, both by experiment and by numerical modeling, of resolution limits for dense nanoscale gratings fabricated in polymethylmethacrylate through low-energy electron-beam lithography (EBL) using 10keV electrons. In particular, they have studied the resist morphologies that develop in various exposure regimes for gratings with periods from 20to100nm and categorized the potential sources of resist damage in such gratings. Through comparison of their experimental and numerical results, they have elucidated the major mechanisms that limit the EBL process resolution at the stages of exposure and resist development. The authors have also suggested semiempirical criteria to predict the risk of resist damage when fabricating dense nanostructures.

Transport study of a single bismuth nanowire fabricated by the silver and silicon nanowire shadow masks

The authors have carried out measurements of the electrical conductivity of single bismuth nanowires fabricated by the low energy electron beam lithography using the silver/silicon nanowire shadow masks. The examined nanowires had cross-sectional dimensions of 40×30 and 40×50nm2. The chosen nanowire sizes had been slightly below the critical diameter D (∼50nm) at which a semimetal to semiconductor phase transition was predicted to occur. The results reveal a semiconductorlike temperature dependence of the electrical conductivity of a bismuth nanowire, which is strikingly different from that of the bulk bismuth.

Fabrication of patterned organic thin film by low-energy electron beam lithography and surface-initiated ring-opening metathesis polymerization

High densities of immobilized polymer brushes have been created on solid supports in a spatially addressable fashion. Octadecyltrichlorosilane was self-assembled on a silicon substrate to form an inert monolayer. The substrate was then patterned by low-energy electron beam lithography. Finally, the exposed region was back-filled with a second functionalized silane and the pattern was further amplified by surface-initiated ring-opening metathesis polymerization. The patterned substrate was imaged by scanning electron microscopy and atomic force microscopy.Key words: patterned thin film, e-beam lithography, ring-opening metathesis polymerization, polymer brushes.

Substrate effects in poly(ethylene glycol) self-assembled monolayers on granular and flame-annealed gold

Poly(ethylene glycol) (PEG) self-assembled monolayers (SAMs) are surface coatings that efficiently prevent nonspecific adhesion of biomolecules to surfaces. Here, we report on SAM formation of the PEG thiol CH 3O(CH 2CH 2O) 17NHCO(CH 2) 2SH (PEG 17) on three types of Au films: thermally evaporated granular Au and two types of Au films from hydrogen flame annealing of granular Au, Au(111), and Au silicide. The different Au surfaces clearly affects the morphology and mechanical properties of the PEG 17 SAM, which is shown by AFM topographs and force distance curves. The two types of SAMs found on flame-annealed Au were denoted “soft” and “hard” due to their difference in stiffness and resistance to scratching by the AFM probe. With the aim of nanometer scale patterning of the PEG 17, the SAMs were exposed by low energy (1 kV) electron beam lithography (EBL). Two distinctly different types of behaviour were observed on the different types of SAM; the soft PEG 17 SAM was destroyed in a self-developing process while material deposition was dominant for the hard PEG 17 SAM.

Three dimensional HSQ structures formed using multiple low energy electron beam lithography

A method is presented for the fabrication of three-dimensional (3D) structures formed in hydrogen silsequioxane (HSQ) by a process of multiple low energy electron beam exposures with a single development step. Structures have been produced consisting of multiple layers of dielectric rods with widths and heights of 150 nm and a separation of 1μm in the horizontal plane and 360 nm in the vertical direction.

Low-energy electron-beam lithography of hydrogen silsesquioxane

Low-energy electron-beam lithography using hydrogen silsesquioxane (HSQ) as a negative electron resist has been investigated in the energy range between 2 and 20 keV. It is found that the required electron dose is drastically reduced at low electron energies and the density of the pattern strongly depends on the concentration of the developer and exposure energy at low keV region. With the tetramethyl ammonium hydroxide (TMAH) developer at concentration of 2.5%, we achieved 12 nm lines of 50 nm period grating with 50 nm thickness of HSQ at 10 keV.