Masked Ion Beam Lithography Research Articles

High-brightness source of energetic He atoms

In atom beam lithography, a beam of kilo-electron-volt helium atoms illuminates a stencil mask and transmitted beamlets transfer the mask pattern to resist on a substrate. It shares the advantages of masked ion beam lithography but is immune to charging artifacts that limit resolution and pattern fidelity. This paper describes a high-brightness source of energetic He atoms, where He+ ions are extracted from a multicusp ion source, focused by two-stage accelerating optics, and neutralized by charge-transfer scattering in a differentially pumped, He-filled cell. Since scattering angles are extremely small, the straight line trajectories of scattered atoms are essentially tangent to the (possibly curved) trajectories of the parent ions. Space-charge repulsion prevents the ion beam crossing over; instead, it converges to a waist of minimum cross section before diverging further downstream. Atom trajectories produced by a cell placed in the region of intense space charge near the waist are strongly affected by the curvature of ion trajectories within the cell. The flaring of the ion beam due to space charge can be used to increase the width of the atom beam, although to the detriment of resolution. In this paper, the authors study a configuration where the cell is placed in the converging ion beam as far as practicable from the ion-beam waist. The atom beam then converges to a crossover, which becomes the virtual source seen by the mask. The source diameter and angular flux density initially increase with increasing cell pressure but saturate at higher pressures; the respective saturation values at 50 keV are 125 μm (2σ) and 8.7 × 1017 particles/s sr. Under these conditions, the beam diameter is ∼2.5 cm, 7 m from the source. A practical system for subnanometer printing is discussed with 0.2 nm (2σ) penumbral blur and 1.25 × 1013 particles/s cm2 flux density over a 1 cm circular field.

Systematic Modification of Electrical and Superconducting Properties of YBCO and Nano-Patterning of High-Tc Superconducting Thin Films by Light-Ion Irradiation

Abstract Irradiation of high-temperature superconducting (HTS) YBa2Cu3O7-d (YBCO) thin films with 75 keV He + ions leads to a quasi-exponential increase of the in-plane (ρab) and the out-of-plane (ρc) resistivity in the normal state and to a non-linear decrease of the critical temperature Tc with ion dose. In situ electrical measurements at room temperature reveal an irradiation-induced reduction of resistivity anisotropy ρc/ρab and a slight relaxation of film resistivity after the ion irradiation is stopped. Ex situ measurements show a stretched-exponential relaxation of Tc and normal state resistivity that continues for several weeks after the ion irradiation. Irradiation of YBCO thin films by low-energy He + ions through stencil masks results in local modification of the electrical and superconducting properties of the HTS material. We demonstrate that masked ion-beam lithography enables to produce structures smaller than 100 nm in size that have potential for applications in future superconducting nano-electronics.

Line edge roughness investigation on chemically amplified resist materials with masked helium ion beam lithography

We conducted line edge roughness (LER) measurements on resists with various sensitivities, exposed with a 75 keV 1:1 masked ion beam lithography tool. The critical dimension measurement data were treated with an algorithm for separation of mask induced roughness from random LER. The scaling analysis approach provided the correlation length and the roughness exponent. The results indicate that for exposure doses >2.5 μC/cm2 LER is not governed by shot noise but by the resist material properties (sensitivity, molecular weight, acid diffusion length) and development conditions.

Line edge roughness investigation on chemically amplified resist materials with masked helium ion beam lithography

We conducted line edge roughness (LER) measurements on resists with various sensitivities, exposed with a 75 keV 1:1 masked ion beam lithography tool. The critical dimension measurement data were treated with an algorithm for separation of mask induced roughness from random LER. The scaling analysis approach provided the correlation length and the roughness exponent. The results indicate that for exposure doses >2.5 μC/cm 2 LER is not governed by shot noise but by the resist material properties (sensitivity, molecular weight, acid diffusion length) and development conditions.

Micro/nanomachining of Polymer Surface for Promoting Osteoblast Cell Adhesion

Interactions between cells and biomaterials are affected by surface properties. Therefore, various approaches have been introduced for surface modifications. Here a technique based on ion beam lithography to improve osteoblast cell adhesion on polymeric materials is reported. We have demonstrated that exposing the polymer to P+ or Ar+ ions through masks can generate micro/nano-scale patterns. Our results illustrate that after exposure to an ion beam, the amount of osteoblast cells attached to the polymer was enhanced as a consequence of the roughened surface as well as due to the implanted ions. This indicates that masked ion beam lithography (MIBL) can not only generate nanostructures on the surface of a biocompatible polymer, but can also selectively modify the surface chemistry by implanting with specific ions. These factors can contribute to an osteogenic environment.

Micro/nano machining of polymeric substrates by ion beam techniques

Ion beam lithography was studied as an alternative approach for micro/nano machining of polymeric substrates. Our preliminary study was carried out using poly(methyl methacrylate) (PMMA) as a model system. Spin cast PMMA films were submitted to masked Ca + ion implantation (85 keV, 1×10 14 ions/cm 2), and P + ion implantations (85 keV, 1×10 15 and 1×10 16 ions/cm 2 respectively). The patterns generated on the films were characterized by AFM as arrays of holes with nanoscale depth and microscale width. The effect of ion implantation on surface property was investigated by measuring surface hydrophobicity. The results showed that masked ion beam lithography can change not only the surface topography, but also the surface hydrophobicity.

Resist process development for sub-100-nm ion projection lithography

Ion projection lithography (IPL) is designed to realize resist structures on the wafer plane well below 100 nm. Patterns from a stencilmask are printed with 4× reduction using an electrostatic lens system. First exposures with the IPL Process Development Tool [Proc. SPIE 3997 (2000)] will start within the fourth quarter of this year. In the meantime, a baseline resist process for i-line, DUV and e-beam resists has been established with exposures on a 1× masked ion beam lithography (MIBL) tool at IMS-Vienna . The results as obtained with 75-keV He + ions are compared with experiments using a 30-keV e-beam writer at IMS-Chips, Stuttgart. Resists were provided by Shipley and Clariant. For the first time, through close interaction with industrial resist companies, ion beam resist materials are available and characterized with well tailored sensitivities in order to vary the IPL exposure dose between 0.4 and 3 μC/cm 2 (double the dose to clear large areas value). These materials will be essential to study and establish the required IPL resist exposure dose for sub-100-nm structures. We measured the sensitivities (dose to clear large areas), contrast curves as well as MIBL dose latitudes. With higher precision than previously done [J. Vac. Sci. Technol. B15 (1997) 2355] the contrast curves have to be established as input data for IPL simulations. The IPL sensitivity is lower by more than an order of magnitude, compared to 30-keV e-beam exposure. Our paper presents and discusses the results. For IPL resist simulations the Monte Carlo simulator SRIM 2000 [Ziegler et al., The Stopping and Ranges of Ions in Solids, Pergamon, New York (1985)] is used to obtain energy density distributions in C x H y O z resist materials. We are presenting and comparing point spread functions of 60 and 75 keV H + ions, He + ions and electrons.

A Versatile Approach for Biomaterial Patterning: Masked Ion Beam Lithography

AbstractWe describe a new approach for biomaterial patterning, viz, masked ion beam lithography. Poly (methyl methacrylate) (PMMA) film was used as a model system and subjected to Ca+ and P+ ion implantations through masks. Ca+ ion implantation was performed at an energy of 85 keV with a fluence of 1×1014 ions/cm2. P+ ion implantation was done at an energy of 85 keV with fluences of 1×1015 and 1×1016 ions/cm2. Arrays of holes were generated during these processes. AFM showed that the depth of the holes is in the nanoscale region. The surface hydrophobicity of the exposed PMMA films was investigated by contact angle measurement. The results indicated that ion implantation changed the surface hydrophobicity.

Masked ion beam lithography with highly charged ions

Masked ion beam lithography using highly charged ions is demonstrated for the first time by producing an array of hundreds of ordered micrometer wide dots using Xe44+ on poly(methylmethacrylate) resist. At low dose, exposure of the resist is incomplete and isolated single-ion impact sites can be seen within the exposed areas. Atomic force microscope images of the single-ion impact sites show craters with a width of 24 nm. At high dose, the exposure is complete and the dot morphology is consistent with limitations from the mask. Scanning electron microscope images indicate that the sidewall slope is steeper than four.

Deep ultraviolet resists AZ DX-561 and AZ DX-1300P applied for electron beam and masked ion beam lithography

This article reports work on two positive-tone deep ultraviolet chemically amplified resist (CAR) systems from HOECHST AG that were applied in two different lithographic processes: (i) the AZ DX-561 resist in serial working direct-write scanning electron-beam lithography, and (ii) AZ DX-1300P resist in a novel parallel working masked ion beam lithography. The main parameters of both CARs under the optimized processing conditions for both lithographic processes were generally determined. The contrast of these resists was found to be >10. The sensitivity of AZ DX-561 was about 12 μC/cm2 for 30 keV electrons. For AZ DX-1300P, a sensitivity of 9 μC/cm2 was found for 30 keV electrons and 0.3 μC/cm2 for 80 keV He+ ions. Both resists showed excellent performance for subsequent reactive ion etching.

Application of optical filters fabricated by masked ion beam lithography

Masked ion beam lithography (MIBL) was employed to fabricate optical filters as a critical component of an energy conversion system which utilizes semiconductor photovoltaics. This article will describe the operation and novel application of these devices and the MIBL pilot production line being facilitized. The conversion concept, thermophotovoltaics (TPV), when coupled with these MIBL produced bandpass filters, is capable of converting heat to electrical power with >20% conversion efficiency. The EDTEK TPV filter is based on a high density array of slotted antenna elements patterned into a single layer of thin gold film.

Masked ion beam lithography for proximity printing

Optical and X-Ray proximity printing systems are resolution limited by diffraction and beam dispersion. Parallel dispersion free ion beam systems are therefore ideal to transfer stencil mask patterns onto all sorts of nonideal substrates. A feasibility study was performed with the existing Alpha ion projector of the Society for the Advancements of Microelectronics in Austria operated in the MIBL (Masked Ion Beam Lithography) mode with ≈ 10×10 mm2 exposure field. Structures as small as 0.2 μm in diameter could be transfered even with a gap of 1 mm between stencil mask and substrate. The widening of resist lines with 10% increase in dose was evaluated to be 14 nm for 2800 μm gap and 4 nm for 300 μm gap. This excellent exposure latitude favourably compares with synchrotron based X-ray lithography, where a widening of 20 nm with 10% overexposure has been reported for a 40 μm gap, and 10 nm for 10 μm gap. Promising applications of the MIBL technique include the fabrication of flat panel displays based on vacuum electronics (field emitter displays), surface acoustic wave and microoptic devices and - in combination with reactive ion etching - the fabrication of micro electro mechanical systems (MEMS). Prospects for MIBL steppers of printing fields > 100×100 mm2 are discussed.

Silicon stencil masks for masked ion beam lithography proximity printing

Silicon membranes with 2 μm to 6 μm thickness and ≈ 10×10 mm2 mask field have been fabricated with the help of electrochemical etch stop techniques. The Si foil was coated with 0.3 μm thick PECVD Si3N4. Shaped electron beam lithography was done in ARCH (OCG) positive resist. RIE etching into the nitride layer was done with CHF3/Ar/SF6. Silicon trench etching was based on Cl2/Ar/BCl3 plasma chemistry implementing gas chopping. Ion beam proximity printing of the Silicon stencil mask structures was done with 55 keV Helium ions into 0.4 μm thick AZ PN114 negative resist using the Alpha ion projector of the Society for the Advancements of Microelectronics in Austria in the MIBL (Masked Ion Beam Lithography) mode. Pattern transfer of a mask feature of less than 100 nm diameter (25:1 aspect ratio in the stencil mask) could be demonstrated even for a mask to wafer gap of 1 mm. The prospects of fabricating large area (> 100×100 mm2) Silicon stencil masks for MIBL printing of gate levels for FieldEmitterDisplays (FEDs) andMicro-Electro-Mechanical-Systems (MEMS) is discussed.

Resolution limits of ion milling for fabricating Y1Ba2Cu3Ox nanostructures

A process is described for fabricating 0.23-μm-wide lines in Y1Ba2Cu3Ox thin films where patterns are defined in a commercial, negative tone, epoxy-based resist by masked ion beam lithography and transferred to the superconducting film by argon ion milling. Lines in 80-nm-thick films had the same zero-resistance temperature (89 K) as the starting films, and a critical current density of 0.7×106 A/cm2 at 77 K, representing a threefold reduction from the starting value. A consistent interpretation of these results is that the line consists of a superconducting core 70 nm in width with the critical current density of the starting film and with 80-nm-wide nonsuperconducting sidewalls. The results were reproducible in lines which did not cross outgrowths in the superconducting film.

The application of channeling to masked ion beam lithography

Various orientations of undoped single crystal silicon were examined from the standpoint of transmitted half-angle to determine the best orientation for masked ion beam lithography (MIBL). The effect of boron, present at a level of ∼ 10 20/ cm 3, was also determined. Transmitted angular distributions, for particles of incident energy in the range 80–180 keV, were measured using a moveable silicon surface barrier detector with a resolution of ∼ 3 keV and an acceptance angle of < 0.1°. Planar orientations yielded smaller transmitted half-angles than axial orientations without a significant loss in transmitted yield. The half-angle of particles transmitted along planes, unlike axes, was found to vary with azimuthal angle. The best orientation for use in MIBL was determined to be the {110} planar orientation. Half-angles for particles transmitted along this plane were smallest for scans perpendicular to the plane and largest for scans parallel to it. The presence of boron was found to increase the transmitted half-angle by ∼ 10% over the entire energy range.

Alignment for masked ion beam lithography using ion-induced fluorescence

We show that the bombardment of densified chemical vapor deposited SiO2 by hydrogen ions induces fluorescent defects. A proton dose of 6×1014 /cm2 saturates the fluorescence yield at approximately 0.6 photons/100 keV of absorbed proton energy. A beam rocking technique was used to obtain a fluorescence alignment signal corresponding to the registration of a stencil mask pattern with a corresponding SiO2 mark on a wafer. Simulation of an optimized alignment system implies that 50 nm (3σ) registration can be achieved with a 1 s settling time. This direct approach to alignment has the advantages of being simple, relatively insensitive to resist coating of the marks, and compatible with conventional integrated circuit processing.

Dry etching induced damage on vertical sidewalls of GaAs channels

Damage on the vertical sidewalls of narrow GaAs structures has been investigated. The structures consisted of narrow conducting channels between Ohmic pads. The channels were defined by either masked ion beam lithography or electron beam lithography with channel widths varying from 0.08 to 1.5 μm. Reactive ion etching and ion beam assisted etching were used to remove the epitaxial material outside the channels down to the semi-insulating substrates. Conduction in the channels was monitored by measuring current–voltage characteristics between the Ohmic pads. It is found that saturation current depends on the dry etching conditions such as ion energy, etching species, and impurity redeposition. Ion beam etching of GaAs substrates at different angles was used to separate out the effects of the inert ion species of reactive ion etching and ion beam assisted etching. Large increases in leakage current and impurity concentrations were obtained when the sample was mounted in the plane parallel to the direction of the ion beam, indicating that redeposition from fixtures and chamber walls can be significant. Controllable etching with minimal damage in GaAs is obtained by using low ion energy, reactive gases, and a contamination-free environment in the chamber. By optimizing the etching conditions, conduction was observed for channels with mirror-smooth sidewalls and widths as small as 0.08 μm.

Submicrometer Structures Fabricated by Masked Ion Beam Lithography and Dry Etching

Dry etching has been used in conjunction with masked ion beam lithography to fabricate devices with controllable submicrometer dimensions. A new self‐aligned technique used to fabricate MESFET's with 0.15–0.27 μm gate lengths and a new thinning technique used to produce columns with widths less than 10 nm are reviewed. Results on dry etching through stencil masks to obtain linewidths as small as 60 nm are presented.

Pattern transfer by dry etching through stencil masks

Anisotropic profiles and linewidths as small as 60 nm have been controllably achieved using pattern transfer by dry etching through stencil masks. This technique eliminates the conventional polymer resist and lithographic steps and could be useful in obtaining precisely reproducible submicrometer linewidths without variation due to run-to-run variability in resist processing. The stencil masks used in this study were 1-μm-thick SiNx membranes with transmission openings and are similar to those used for masked ion beam lithography. The dry etching techniques consisted of reactive ion etching, ion beam assisted etching, and hot jet etching. The profile and linewidth control depend on the divergence of the ion or reactive flux and the gap between the stencil mask and the sample.

Nanometer-scale columns in GaAs fabricated by angled chlorine ion-beam-assisted etching

Angled chlorine ion-beam-assisted etching has been used in combination with masked ion beam lithography to produce columns in GaAs with widths of less than 10 nm and height-to-width ratios greater than 25. This technique allows the highly controllable fabrication of structures with dimensions smaller than initially defined by the lithography. It can be applied to the fabrication of ultrasmall GaAs/AlGaAs quantum well structures having quantized energy states in two or three dimensions while at the same time being compatible with full-wafer processing.