Parallel Electron Beam Lithography Research Articles

Stacked Integration of MEMS on LSI.

Two stacked integration methods have been developed to enable advanced microsystems of microelectromechanical systems (MEMS) on large scale integration (LSI). One is a wafer level transfer of MEMS fabricated on a carrier wafer to a LSI wafer. The other is the use of electrical interconnections using through-Si vias from the structure of a MEMS wafer on a LSI wafer. The wafer level transfer methods are categorized to film transfer, device transfer connectivity last, and immediate connectivity at device transfer. Applications of these transfer methods are film bulk acoustic resonator (FBAR) on LSI, lead zirconate titanate (Pb(Zr,Ti)O3) (PZT) MEMS switch on LSI, and surface acoustic wave (SAW) resonators on LSI using respective methods. A selective transfer process was developed for multiple SAW filters on LSI. Tactile sensors and active matrix electron emitters for massive parallel electron beam lithography were developed using the through-Si vias.

Fabrication of Pierce‐Type Nanocrystalline Si Electron‐Emitter Array for Massively Parallel Electron Beam Lithography

SUMMARYThis paper reports on the development of a fundamental process for a Pierce‐type nanocrystalline Si (nc‐Si) electron emitter array for massively parallel electron beam (EB) lithography based on active‐matrix operation using a large‐scale integrated circuit (LSI). The emitter array consists of 100 × 100 hemispherical emitters formed by isotropic wet etching of Si. EB resist patterning was demonstrated by 1:1 projection exposure using a discrete emitter array at CMOS‐compatible operating voltages. Isolation trenches filled with benzocyclobutene (BCB) were fabricated in the Si substrate for independent control of each emitter using the LSI. The integration process of the emitter array with LSI and an extraction electrode plate was also developed based on Au‐In and polymer bonding technologies.

17×17並列電子源駆動システムの開発

The 17×17 parallel electron beam lithography system has been implemented and evaluated to clarify its system problems and to accelerate a 100×100 massive parallel electron beam direct draw system under development. This paper describes its system concept, an electron emitter driver circuit, its control system and an exposure result.

Design of electrostatic microcolumn for nanoscale photoemission source in massively parallel electron-beam lithography

Microcolumns are widely used for parallel electron-beam lithography because of their compactness and the ability to achieve high spatial resolution. A design of an electrostatic microcolumn for our recent nanoscale photoemission sources is presented. We proposed a compact column structure (as short as several microns in length) for the ease of microcolumn fabrication and lithography operation. We numerically studied the influence of several design parameters on the optical performance such as microcolumn diameter, electrode thickness, beam current, working voltages, and working distance. We also examined the effect of fringing field between adjacent microcolumns during parallel lithography operations.

超並列電子線描画用LSIの設計と評価

超並列電子線描画用LSIの設計と評価

Fabrication of Pierce-Type Nanocrystalline Si Electron-Emitter Array for Massively Parallel Electron Beam Lithography

SUMMARY This paper reports on the development of a fundamental process for a Pierce-type nanocrystalline Si (nc-Si) electron emitter array for massively parallel electron beam (EB) lithography based on active-matrix operation using a large-scale integrated circuit (LSI). The emitter array consists of 100 × 100 hemispherical emitters formed by isotropic wet etching of Si. EB resist patterning was demonstrated by 1:1 projection exposure using a discrete emitter array at CMOS-compatible operating voltages. Isolation trenches filled with benzocyclobutene (BCB) were fabricated in the Si substrate for independent control of each emitter using the LSI. The integration process of the emitter array with LSI and an extraction electrode plate was also developed based on Au-In and polymer bonding technologies.

Digital pattern generator: an electron-optical MEMS for massively parallel reflective electron beam lithography

The digital pattern generator (DPG) is a complex electron-optical MEMS that pixelates the electron beam in the reflective electron beam lithography (REBL) e-beam column. It potentially enables massively parallel printing, which could make REBL competitive with optical lithography. The development of the REBL DPG, from the CMOS architecture, through the lenslet modeling and design, to the fabrication of the MEMS device, is described in detail. The imaging and printing results are also shown, which validate the pentode lenslet concept and the fabrication process.

(Invited) Ballistic Electron Effects in Nanosilicon and Their Applications

In nanocrystalline silicon (nc-Si) dots interconnected with tunnel oxides, the characteristic electron transport mode appears followed by highly hot electron effects: avalanche photoconduction and quasi-ballistic electron emission. Analysis of the former clarifies that the impact ionization rate in nc-Si dots is considerably higher than in bulk Si. The usefulness of the latter has been confirmed in various media. A typical example of the application in vacuum is the use as the excitation source for parallel electron beam lithography. Another potential is induced by the operation in aqueous and metal salt solutions, where the highly reducing reaction proceeds at the emitter surface. In SiCl4 and GeCl4 solutions, for instance, thin Si and Ge films are deposited without the use of any counter electrodes. Some topics of these effects are discussed here.

Multiaxis and multibeam technology for high throughput maskless E-beam lithography

The authors have devised a new maskless E-beam lithography method which can achieve a throughput of 10 wph which the authors are calling a “multiaxis programmable shaped beam” (multiaxis PSB). It is a kind of massive parallel electron beam lithography wherein more than 250 000 beams write a wafer and the total beam current reaches a hundred microamperes. It may be difficult to write a pattern finer than 10 nm with more than 1 μA beam current at 50 kV with a single column because of blurring from Coulomb interaction, so the multiaxis PSB system divides all the beams into 87 groups where each group has 2500–10 000 beams formed by a small column arrayed in a 26 × 33 mm2 lattice. The small column is called a column element (CE) and each CE has an emitter, apertures, deflectors, and small lenses. The key technologies of the multiaxis system are a lens, emitter, and blankers for individual beamlets. The authors have proposed new ideas for these issues and made fundamental experiments.

Precisely-controlled fabrication of single ZnO nanoemitter arrays and their possible application in low energy parallel electron beam exposure

Precisely-controlled fabrication of single ZnO nanoemitter arrays and their possible application in low energy parallel electron beam exposure are reported. A well defined polymethyl methacrylate (PMMA) nanohole template was employed for local solution-phase growth of single ZnO nanoemitter arrays. Chlorine plasma etching for surface smoothing and pulsed-laser illumination in nitrogen for nitrogen doping were performed, which can significantly enhance the electron emission and improve the emitter-to-emitter uniformity in performance. Mechanisms responsible for the field emission enhancing effect are proposed. Low voltage (368 V) e-beam exposure was performed by using a ZnO nanoemitter array and a periodical hole pattern (0.72-1.26 μm in diameter) was produced on a thin (25 nm) PMMA. The work demonstrates the feasibility of utilizing single ZnO nano-field emitter arrays for low voltage parallel electron beam lithography.

Ballistic electron emission from quantum-sized nanosilicon diode and its applications

A quantum confinement effect renders silicon a functional wide-gap material with useful functions. For instance, a diode based on nanocrystalline silicon (nc-Si) exhibits characteristic quasi-ballistic emission effects in various media. As means for physical excitation and probing, the applicability to parallel electron beam lithography and high-sensitivity image-pickup has been demonstrated in vacuum. The energetic electron incidence into air and Xe ambient induces negative ion generation by electron attachment into oxygen molecules and vacuum ultraviolet light emission by internal excitation of Xe molecules, respectively. Another effect is that the nc-Si ballistic emitter can supply highly reducing electrons into aqueous and metal-salt solutions without the use of counter electrodes. This is an attractive process that will be applicable to hydrogen generation and thin metal film deposition.

Transformation of polycrystalline tungsten to monocrystalline tungsten W(100) and its potential application in Schottky emitters

The electron sources in electron microscopes and electron lithography machines often consist of small diameter W(100) wires, etched to form a sharp tip. The electron emission is facilitated by the Schottky effect, thus the name Schottky emitter. The authors are investigating the feasibility of arrays of such electron emitters for the use in multibeam lithography. This, however, would require large pieces of W(100) which are not easily available. The concept of dc Joule heating for the crystallographic transformation is utilized to convert polycrystalline tungsten to W(100) for the fabrication of the Schottky emitters. A miniature Schottky emitter fabricated by wire electrical discharge machining was heated by dc and the crystallographic orientation was subsequently determined by electron diffraction. The x-ray diffraction measurement on another sample of tungsten filament, heated by similar method, confirms the above results. The concept will be extended for the fabrication of a prototype multibeam source for parallel electron beam lithography.

Sub-50nm resolution surface electron emission lithography using nano-Si ballistic electron emitter

It is demonstrated that parallel electron beam lithography using a nano-Si planar ballistic electron emitter (PBE) provided the resolution below 30nm even in a low accelerating voltage. This high resolving power is explained by the nature of ballistic electron transport in nanocrystalline silicon. The parallel EB lithography was performed on a 1:1 electron imaging system. The system was composed of the PBE as a patterned surface electron source, a target wafer parallel to the surface electron source, and vertical electromagnetic fields. The PBE projected a patterned electron image on the target. A replica of the pattern was transferred into a resist on the target wafer within less than 1s. The experimental exposure was performed over 10mm2 area. The PBE was composed of a thin metal surface electrode, nanosilicon layer, and semiconductor substrate. The electrons injected from semiconductor substrate were accelerated via cascade tunneling through the nanosilicon layer and reached the outer surface as ballistic or quasiballistic electrons. As a result, the chromatic aberration limiting resolution is expected to be quite small. The 1:1 electron imaging system based on the PBE provides promising solution for high resolution and high throughput lithography with low cost system with the simple electron optics for the next generation device fabrication in the sub-30nm scale.

Fabrication of miniaturized Schottky emitter by wire electrical discharge machining (WEDM)

The Schottky emitter is extensively used in scanning electron beam lithography machines because of its high brightness and stable current. For its use in parallel electron beam lithography to incre...

Fabrication of miniaturized Schottky emitter by wire electrical discharge machining (WEDM)

The Schottky emitter is extensively used in scanning electron beam lithography machines because of its high brightness and stable current. For its use in parallel electron beam lithography to increase the throughput, we are investigating the possibility of creating an array of miniaturized Schottky emitters. This paper discusses a novel method of fabricating a miniaturized Schottky emitter of 1mm diameter by wire-electro discharge machining (WEDM). Various WEDM process parameters were optimized for better reproducibility and surface finish of the fabricated emitter. The fabricated samples match well with the intended input design, with no visible stress or bending. Joule heating in vacuum shows good operational feasibility and temperature stability. After heating, the tip becomes as sharp as conventional Schottky emitters. The miniaturized emitter operates at a relatively low power.

Thermal model of miniaturized Schottky emitter for parallel electron beam lithography

For parallel electron beam lithography, an array of miniaturized Schottky emitters is proposed. The design of this miniaturized Schottky emitter unit is analyzed thermally to determine optimum power and filament dimensions. A numerical thermal analysis in steady state condition has been done and the temperature of the tip has been determined for different currents and heating filament dimensions. The model has been verified with some available experimental data. The optimized solution from the model will be used for fabrication of an array of miniaturized Schottky emitters.

Modeling of the integrated magnetic focusing and gated field-emission device with single carbon nanotube

A gated single carbon nanotube field emitter with magnetic focusing is proposed and simulated using a parallelized Poisson’s equation solver, coupled with the ray tracing of electrons, on an unstructured tetrahedral adaptive mesh. The magnetic focusing for the electrons can be achieved by a vertically downward magnetic focusing field (−Bz) through the use of either external solenoids or permanent magnets around the field-emission array. The simulation results, assuming uniform magnetic field inside a field-emission unit, are compared with those conventional tetrode-type field emitters using an electrostatic focusing structure. The results reveal that the magnetic focusing design can promise much higher emission current, while a much smaller spot size results at the anode. In addition, the magnitude of the applied gate voltage in the range of 60–120V shows little influence on the electron-beam diameter at the anode. The proposed magnetic focusing method can also possibly reduce the complexity of the fabrication without the electrostatic focusing structure. Noticeably, a distribution, similar to the Airy function, is obtained that shows the dependence of the spot size at the anode on the magnetic flux intensity. Thus, under suitable magnetic focusing conditions, it is possible to produce well-defined microelectron sources for many field-emission applications, such as novel parallel electron-beam lithography or field-emission displays.

Electron-beam focusing characteristics of double-gated carbon nanofiber based field emission sources

Recently, vertically aligned carbon nanofiber based dual-gate field emission structures have been fabricated for use in parallel electron-beam lithography, and their electron-beam focus adjustment capability has been demonstrated. This article summarizes the results of a numerical investigation of device behavior due to geometry variation in an effort to better understand device characteristics and performance. The effect of electrode thickness on minimum beam diameter and depth of field (DOF) were investigated for axially symmetric structures using a two-dimensional simulation package. The results indicate both a decrease in minimum spot size from ∼30to∼20nm, and an increase in DOF from ∼1to∼6μm when the focus electrode thickness is increased from 100to500nm. The impact of misalignments of the focus electrode and the carbon nanofiber on the beam behavior was investigated with a three-dimensional simulation package. Results show that reasonably well-converged beams can be achieved even with considerable offset to either the focus electrode or carbon nanofiber, though large beam deflections are produced. In addition, the deflections due to these offsets appear to be linear.

Simulations of emission characteristics of a multigated single carbon nanotube field emitter

In this study, a two-dimensional, particle-in-cell computer simulation code MAGIC was used to investigate the field emission characteristics from a single closed-capped multiwalled carbon nanotube (MWCNT) in a particular quadruple-gated focusing configuration. Simulations have been done on a single MWCNT assuming it is a cylinder with an ellipsoidal cap of 40nm major radius and 10nm minor radius, 20nm in base diameter, and 0.2 or 0.3μm in height. Other simulation parameters in the base case are 0.1μm thickness for each gate, uniform gate hole radius of 0.45μm, and an anode-cathode distance of 20μm. A particular quadruple-gated focusing configuration has been investigated with individual gate bottom to cathode top distances of 1.1, 1.8, 2.9, and 3.9μm, respectively. For this particular quadruple-gated structure with a cathode voltage of 0V, 85V in voltage of the first gate, 224V in voltage of the second gate, 1320V in voltage of the third gate, 1331V in voltage of the fourth gate, and an anode voltage of 1331V, simulation result showed that a current-weighted beam spot radius on the anode plane can be reduced to 17.4nm for a MWCNT height of 200nm. The emission current, however, varies only slightly from 0.311to0.375pA, as the voltage on the third gate is changed from 200to1500V. The region in the vicinity of the second gate acts to focus the emitted electron beam, while that of the third gate acts to diverge the beam. It was also found that using a higher MWCNT would reduce the applied voltages, but the emitted electrons could not be as well focused as the case with shorter MWCNT. For a MWCNT height of 300nm at about 0.3pA emission current, the weighted beam radius is increased slightly to 28.8nm at the optimum focusing condition. For the MWCNT height of 300nm at a higher emission current about 3nA, the weighted beam radius was found to increase to 46.4nm at the optimum focusing condition. This study shows that it is possible to design a quadruple-gated MWCNT field emitter having few tens of nanometer focused beam size at picoampere to nanoampere emission current range and the designed field emitter is suitable for parallel electron-beam lithography applications.

Fabrication and electrical characteristics of carbon nanotube-based microcathodes for use in a parallel electron-beam lithography system

This article presents an overview of the “Nanolith” parallel electron-beam (e-beam) lithography approach. The e-beam writing head consists of an array of microguns independently driven by an active matrix complementary metal–oxide–semiconductor circuit. At the heart of each microgun is a field-emission microcathode comprised of an extraction gate and vertical carbon nanotube emitter, whose mutual alignment is critical in order to achieve highly focused electron beams. Thus, in this work, a single-mask, self-aligned technique is developed to pattern the extraction gate, insulator, and nanotubes in the microcathode. The microcathode examined here (150×150 gates, 2 μm gate diameter, with multiple nanotubes per gate) exhibited a peak current of 10.5 μA at 48 V when operated with a duty cycle of 0.5%. The self-aligned process was extended to demonstrate the fabrication of single nanotube-based microcathodes with submicron gates.