Ion Optical Column Research Articles

Experimental results using an improved low-energy focussed ion beam column with booster principle and free-standing target

The design and experimental results achieved with an improved low-energy focussed ion beam (LEFIB) column are presented. The ion optical column is based on booster principle and electrostatic immersion lenses which are operated in the internal acceleration mode. Ion beam retarding is accomplished within the objective lens. Therefore the target is free-standing, i.e. on ground and in a field-free region, and the secondary electrons can easily be detected. The simulated diameters of the high-current gallium ion probe including the important underlying physics of Coulomb interaction have been confirmed in experiment, e.g. 780nm at 1keV landing energy, 1nA beam current and a free working distance of 10mm.

Design of practical deflection field in nanometer-scale focused ion beam system

Two pre-lens deflectors were optimized to reduce the deflection chromatic aberration in a focused ion optical column. Correction principles of the dynamic focus lens and stigmator were researched in detail. The deflection field curvature, astigmatism and distortion can be eliminated by superposing suitable signals on the objective lens and deflectors, which effectively reduces the complexity of the ion optical column. The optical properties of the two deflectors with dynamic correction were simulated and their power supply circuit was built based on superposition theory of electric signals. When the beam current is 1 nA, the pre-lens deflectors can obtain nanometer-scale focused ion beam at the corner of 0.2×0.2 mm 2.

Demonstrations of electronic pattern switching and 10× pattern demagnification in a maskless microion-beam reduction lithography system

A proof-of-principle ion projection lithography (IPL) system called maskless microion-beam reduction lithography (MMRL) has been developed and tested at the Lawrence Berkeley National Laboratory for future integrated circuits manufacturing and thin-film media patterning [V. V. Ngo et al., J. Vac. Sci. Technol. B 17, 6 (1999)]. This MMRL system is aimed at completely eliminating the first stage of the conventional IPL system [G. Stengl et al., J. Vac. Sci. Technol. B 10, 2824 (1992)] that contains the complicated beam optics design in front of the stencil mask and the mask itself. It consists of a multicusp rf plasma generator, a multibeamlet pattern generator, and an all-electrostatic ion optical column. Results from ion beam exposures on poly(methymethacrylate) and Shipley UVII-HS resists using 75 keV H+ are presented in this article. Proof-of-principle electronic pattern switching together with 10× reduction ion optics (using a pattern generator made of nine 50 μm switchable apertures) has been performed and is reported in this article. In addition, the fabrication of a microfabricated pattern generator [K. L. Scott et al., J. Vac. Sci. Technol. B 18, 6 (2000)] on a silicon on insulator membrane is also presented.

Reference plate manufacturing process for the ion projection lithography pattern lock system

The 4×Ion Projection Lithography (IPL), which is designed to reach sub 70-nm feature sizes is a promising technology for the Next Generation Lithography (NGL). An important feature of IPL is the ‘pattern lock’ system allowing on-line control of the reduction ion image of stencil mask patterns projected onto the resist coated wafer substrate. An important part of the pattern lock system is a ‘reference plate’ which is attached to the last lens electrode of the ion-optical column. A process flow for the fabrication of this reference plate out of Si and Zerodur has been developed. This paper describes the Si device manufacturing process. First results of LMS IPRO placement measurements are presented and discussed.

Ion projection lithography: Status of the MEDEA project and United States/European cooperation

Structure and targets of the European MEDEA project on ion projection lithography as well as related U.S./European cooperation are explained. By assuming 10 μm virtual source size and 1 eV (full width half maximum) energy spread calculations for a multielectrode electrostatic ion–optical system (1.25 m between ion source and stencil mask, ≈1.8 m between mask and wafer) we realize the possibility of 100 nm resolution (line and space) over an exposure field of 22×22 mm2 even when using the MONTEC model for calculating the stochastic blur and when running 3.3 μA He+ ion beam current through the ion–optical column, thus more than twice exceeding target specifications. Thus, for 100 nm resolution and 50% pattern density the raw throughput is ≈12 cm2/s corresponding to >75 WPH (pattern within 80% of 300 mm wafer area).

Programmable aperture plate for maskless high-throughput nanolithography

One of the most serious challenges to extending lithography (by whatever radiation) into the sub-100 nm regime is mask manufacturing technology. We propose and analyze a pattern writing concept somewhat like a dot matrix printer that can expose resist with 25 nm pixels at rates needed for production lithography. It consists of a programmable aperture plate which defines an array of 3000×3000, 5 μm×5 μm apertures on 20 μm centers which is illuminated by a collimated beam of either ions or electrons. Each aperture can be blanked off by suitable electrodes and is individually addressed. The image of the array is demagnified on the wafer by 200×. The pattern is entered into the array from one edge only and is toggled across the array by shift registers. The wafer is moved in synchronism. Each pixel on the wafer is exposed 750 times by 750 different pixels to accumulate the total dose. If ions are used (e.g., H+) and a total maximum current of 3 μA is passed down the ion optical column, consistent with space charge limits, then the writing speed with 25 nm×25 nm pixels on the wafer is 1 cm2/s for an ion resist sensitivity of 3 μC/cm2. The aperture plate can be fabricated from a sandwich of 3 Si membranes and will exploit the high aspect ratio etching of 〈110〉 Si to achieve the structures that hold the blanking plates. The aperture array will measure 6 cm×6 cm. The shift register circuits clocking at about 10 MHz will be built in the spaces between the apertures and are feasible with state of the art devices.

Analysis of stencil mask distortion in ion projection lithography

Accurate feature placement on the wafer in ion projection lithography requires that the distortion due to stress relief produced by the pattern of holes in the mask be controlled. We have used the finite element method provided by the ANSYS software package to calculate the two dimensional mask distortion due to non-symmetrical patterns of openings in stencil masks and to analyze a method for controlling distortion. The ion projection lithography mask is a circular silicon membrane 120 mm in diameter 2.5 μm thick rigidly held around the perimeter. The central 60 × 60 mm square area will contain the pattern to be printed, demagnified by 3x. We take the intrinsic stress of the membrane to be 10 MPa, the Young's modulus of the membrane to be E = 1.5 × 1011 Pa and the Poisson ratio ν = 0.17. We have explored the x-y distortion produced by various simple geometries. For example, consider an asymmetric grid of square holes which is 22% open on one half and 10% open on the other half. High distortion in this case will occur along the diameter, between the two halves. The boundary between the two halves (along the diameter) is bowed by 182 nm.A method for reducing the distortion is to cut a pair perforation rings along the perimeter. The perforation rings will have the effect that the central part of the membrane will have the stress relieved and it will be held in effect by “springs” at a constant, low stress level. We have calculated that the bow of the diameter can be reduced to 18 nm by a suitable geometry of the perforations. The perforation ring is an effective means of reducing distortion to well below an acceptable level. The uniform shrinkage of the pattern due to stress relief is simply a small change in magnification which is automatically corrected by the beam lock in the ion optical column.

Global and stochastic space-charge effects in ion beam lithography

Ion projection lithography (IPL) is a candidate technology for meeting expected circuit design rules (less than 0.18 μm) for future generations of semiconductor devices. The Advanced Lithography Group (ALG), a consortium of industrial and U. S. government laboratories and universities, is developing IPL technology with funding from the Advanced Research Projects Agency (ARPA) and the state of Maryland. A prototype IPL device, the ALG-1000, is being developed to demonstrate the capability of IPL to meet future requirements for pattern overlay. To realize IPL technology requires control of space charge effects in the ion optical column. Due to the length of the IPL system (several meters), the precision of the calculation to predict distortion at the wafer plane becomes difficult to perform. Including the effects of the space charge is even more difficult. Both global and stochastic space-charge phenomena occur. This article presents a system of computer models that allows simulations of both global and stochastic space-charge effects. In particular, the models will be used on the ion beam projector in the ALG-1000 device. The calculations are carried out using a self-consistent equilibrium ray tracing code, where the applied fields are calculated from the actual lens column geometry. For global space charge, the model also includes optimization of the lens electrode voltages to minimize pattern distortion at the wafer plane.

Evaluation of critical design parameters of an ion projector for 1 Gbit DRAM production

The Advanced Lithography Group (ALG) a consortium of industry and university institutes in the US and Europe has the aim to produce an Ion projection lithography tool ALG-1000 for 0.18 μm geometry and below. The base of the machine is a new ion optical column developed by IMS, Vienna for exposing 20 mm × 20 mm fields at 3x reduction. The errors of the first two lenses (diverging lens and field lens) are fully compensated in 3rd order by an asymmetric Einzellens projecting an ion image of the stencil mask onto the wafer substrate with better than 2 mrad telecentricity. Remaining are errors of 5th order resulting in minimum calculated distortions of less than 30 nm. Space charge effects have been investigated with high transparency masks in two different ion projectors. An indication for a space charge effect was observed in the case of the Alpha 5x system with a crossover energy of 7.5 keV if the beam current was raised above 10 nA. The experiments with the IPLM-02 system with a crossover at approximately 50 keV, more representative for the conditions in the ALG-1000 with a crossover energy of 200 keV, allowed the resolution of 0.18 μm lines and spaces even at high currents of up to 2 μA.

Bakeable duoplasmatron ion gun for SIMS microanalysis

A 10-keV duoplasmatron ion gun with ion optical column is presented. The system is fully bakeable; copper gaskets and alumina isolators are used. The source can operate with hot filament or hollow cathode. The differentially pumped ion optical column consists of a set of three lenses, double octupole stigmator, deflection-scanning system, and diaphragm set.

A focused gas-ion beam system for submun application

Our gas field ion source, equipped with a supertip, has been complemented with an ion-optical column in order to produce a finely focused beam. The rather simple optical system consists of two einzel lenses of the Riddle type, an x− y steerer pair and a 12-pole deflector after Bonshtedt. Our calculations indicate that a limiting current of about 10 nA can be delivered into a 100 nrn image spot, corresponding to a current density of 100 A/cm 2, when previously measured ion source currents at a transmission of 75% are assumed. The presently obtained values are still considerably lower due to several technical limitations which are being overcome by a few slight modifications of the mechanical construction. The smallest image spot observed so far has been limited to 0.25 μm due to mechanical vibrations. First applications of submun beams will be reported and further plans on both material analysis and modification discussed.

Focused ion beam system with high current density

The project of the IMSA-100 focused ion beam system is presented. The main goal of the system is to achieve an ion current density >10 A/cm2. The ion-optical column consists of a liquid metal ion source, two electrostatic lenses, a pre-lens octupole deflector, E∗B mass separation system, beam blanking plates, and two octupole stigmators. A lithium ion source has been developed.

Micromachining with focused ion beams

Abstract Focused ion beam (FIB) systems are being increasingly used for many highly demanding fabrication applications in nanotechnology, yet so far little attention has been given to the associated problems of in situ metrology. Here, a FIB system is described which consists of an ion optical column, computer-controlled specimen stage, particle detector and image-processing hardware. The FIB column provides a minimum probe size of 250 nm at a variable working distance of 30–40 mm. The maximum field of view is 4 × 4 mm at a working distance of 40 mm. Beam current density is 100 mA cm −2 . The instrument can be operated in micromachining mode for ultra-high precision material removal or in imaging mode for scanning ion microscopy. The use of FIB milling in microcircuit modification and repair, microsensor fabrication and other nanotechnology applications is described. Preliminary data are presented on the deposition of metal patterns with high spatial resolution.

A Scanning Ion Mierobeam System with a Liquid Metal Ion Source

A versatile scanning ion microbeam system using a liquid metal ion source and a one or two lens version ion optical column is developed. It is well suited for testing individual ion optical elements and for simple mieroprobe applications. Beams of 6 to 30 keV Ga+ ions of smaller than 1 μm diameter with 300 pA current are produced.

The application of liquid metal ion sources to SIMS

The high brightness of liquid metal field emission sources makes them uniquely suited to the production of mufocused ion beams. A gallium liquid metal ion source has been succesfully incorporated in a commercial ion-probe system (V G SIMSLAB). The source is fitted to an electrostatic ion-optical column with two einzel lenses, scan rods and quadrupole stigmators: a condenser lens allows the final probe current to be set in the range 50 pA–100 nA, while a final lens allows a minimum probe diameter approaching 0.1 μm at 10 keV energy (at | probe ≈ 50 pA). Ion energy can be varied from less than 1 to 10 keV while maintaining stabilized emission from the source. Secondary ion mass spectra and images have been obtained using gallium ions from a wide range of materials (semiconductors, catalysts and metals). High secondary yields for Glds for Ga + ions allow TV rate display of secondary electron images, detected using a scintillator and photomultiplier: using a quadrupole mass spectrometer secondary ion maps are obtained in typically 40–300 s. The secondary ion maps show the same limiting spatial resolution as the secondary electron images, and can be readily obtained from insulating samples using low energy electrons to neutralize surface charging.