Extremely high-aspect-ratio patterns in macroporous substrate by focused-ion-beam etching: the realization of three-dimensional lattices

We show the very particular behavior of focused-ion-beam etching in macroporous silicon. We demonstrate that, contrary to bulk samples, a porous substrate allows extremely high-aspect-ratio patterns to be etched at submicrometer scales. Thanks to the pre-introduced porosity, the secondary effects that limit the pattern depth in bulk-sample etching, namely the sputtered material redeposition as well as the beam ‘self-focusing’ effects, are strongly reduced in a porous sample. In this case the walls between the pores are sputtered in an almost independent way. The etching of deep and straight patterns is feasible. Combined with photoelectrochemical etching that generates the initial macropores, three-dimensional (3D) lattices can be obtained, as demonstrated by 3D photonic crystal fabrication. Received: 21 August 2002 / Accepted: 21 August 2002 / Published online: 12 February 2003 RID="*" ID="*"Corresponding author. Fax: +33-1/6915-6086, E-mail: wang@lps.u-psud.fr     

Processing of vacuum microelectronic devices by focused ion and electron beams

Localized physical and chemical reactions induced by focused ion and electron beams, i.e. dual beams, have been used to fabricate field emitters (FEs) and their arrays, field-emitter arrays (FEAs), without masking and annealing processes. Issues arising from beam processing such as beam-induced damage and contamination were eliminated to provide FEAs with low leakage current. Quick prototyping and repairing processes of FEs and FEAs using dual-beam processing have been demonstrated. Nb- or Au-gated Pt FEAs have been fabricated using dual beams. The fabricated FEAs showed a turn-on voltage of 40 V for field emission with a typical emission current of about 1 μA/tip. Received: 21 August 2002 / Accepted: 21 August 2002 / Published online: 12 February 2003 RID="*" ID="*"Corresponding author. Fax: +81-6/6850-6662, E-mail: takai@rcem.osaka-u.ac.jp     

Fabrication of Step-and-Flash Imprint Lithography (S-FIL) templates using XeF<Subscript>2</Subscript> enhanced focused ion-beam etching

The fabrication of Step-and-Flash Imprint Lithography (S-FIL) templates with line widths of 50 nm is described in this work. The structures have been patterned using a Ga⁺ focused ion beam (FIB) in a quartz template. FIB milling is generally accompanied with re-deposition effects, which represent a hindrance to densely patterned nanostructures required in most NIL applications. To reduce these re-deposition effects, in this research, xenon difluoride (XeF₂) enhanced FIB etching was applied that also increases the material removal rates in comparison to pure kinetic ion sputtering. To optimise the process when using XeF₂ gas the following ion scanning parameters have been examined: ion dose, beam current, dwell time and beam overlap (step size). It has been found that the assisting gases at very low doses do not bring significant etching enhancements whilst the sputtering rates have increased at high doses. Using the XeF₂ gas-assisted etching, FIB structuring has been used to fabricate <100 nm structures onto quartz S-FIL templates. The presence of XeF₂ considerably enhances the etching rate of quartz without any significant negative effects on the spatial resolution of the FIB lithographic process and reduces the template processing time.     

Transient-diffusion effects

Transient effects on diffusion and activation during post-implantation anneals are a major obstacle for the further miniaturization of ultra-large-scale integrated semiconductor devices. The article reviews recent developments in the simulation of such phenomena with particular emphasis on models for the kinetics of self-interstitial agglomerates and boron–interstitial clusters. Received: 21 August 2002 / Accepted: 21 August 2002 / Published online: 12 February 2003 RID="*" ID="*"Corresponding author. Fax: +49-9131/761-212, E-mail: pichler@iis-bfhg.de     

Three-dimensional structuring of sapphire by sequential He+ ion-beam implantation and wet chemical etching

We present a method for the selective two- and three-dimensional patterning of sapphire using light ion-beam implantation to generate severe lattice damage to depths exceeding 1 μm and subsequent selective wet chemical etching of the damaged regions by hot H₃PO₄. C-cut sapphire crystals were implanted through contact masks using ion fluences of 1×10¹⁶ to 5×10¹⁷ He⁺/cm² and energies up to 400 keV. The etching process is characterized by a high selectivity and a rate of approximately 19 nm/min. Whereas an implantation that produces a continuously damaged pathway results in complete etching from the surface, sole in-depth implantation using only high-energy ions leads to under-etching of the crystalline surface layer. By a combination of these processes we have fabricated three-dimensional structures such as channels and bridges in sapphire. Received: 14 October 2002 / Accepted: 15 October 2002 / Published online: 26 February 2003 RID="*" ID="*"Corresponding author. Fax +41-21/693-3701, E-mail: aurelian.crunteanustanescu@epfl.ch     

Energy and mass dependence of isotopic enrichment in sputtering

Silver, copper, and boron (from a boron nitride target) were sputtered with xenon ions. The isotopic composition of secondary ions of silver was measured at ion energies ranging from 300 eV to 3 keV and, for copper and boron, at 2.0, 2.5, and 3.0 keV. An ion gun was used to generate the ion beam. The secondary ions were detected at a small emission angle by a quadrupole mass spectrometer. The secondary-ion flux of silver was found to be enriched in heavy isotopes at lower incident-ion energies. The heavy-isotope enrichment was observed to decrease with increasing primary-ion energy. Beyond 500 eV, light isotopes of silver were sputtered preferentially with the enrichment increasing to a constant value of 1.018. The sputtered flux of copper and boron also indicated constant enrichments (1.008 and 1.281 for copper and boron respectively) in light isotopes at high ion energies. Received: 2 August 2002 / Accepted 9 August 2002: / Published online: 4 December 2002 RID="*" ID="*"Present address: Pacific Northwest National Laboratory, Richland, WA 99 352, USA RID="**" ID="**"Present address: Philips Display Components Company, Ottawa, OH 45 875 USA RID="***" ID="***"Corresponding author. Fax: +1-334/727-8090, E-mail: pkray@tusk.edu     

Laser-induced etching and deposition of W using a-SiO<Subscript>2</Subscript> microspheres

A regular lattice of a-SiO₂ microspheres on a quartz support is used as a microlens array for laser-induced surface patterning by etching and deposition of W in atmospheres of WF₆ and WF₆+H₂, respectively. Received: 22 July 2002 / Accepted: 30 July 2002 / Published online: 4 December 2002 RID="*" ID="*"Corresponding author. Fax: +43-732/2468-9242, Email: dieter.baeuerle@jku.at     

Stress anisotropy in circular planar magnetron sputter deposited molybdenum films and its annealing effect

As part of the program to develop a free-standing thin-film filter for soft X-ray optics application, stress anisotropy in the molybdenum films deposited by dc circular planar magnetron sputtering were studied by X-ray diffraction (XRD) as a function of sputtering argon gas pressure over a range of 0.8–1.5 Pa. Surface morphology of the films has been investigated by optical microscopy and scanning tunneling microscopy (STM). It is found that, for the film deposited at 0.8 Pa pressure, the stresses are more compressive in the tangential than in the radial direction; the highest compressive stress exists in the center area. The film deposited at 1.5 Pa pressure has the highest stress anisotropy, and the stresses are less tensile in the tangential than in the radial direction. Annealing in vacuum is more effective in reducing tensile stress and stress anisotropy in the tensile stressed film than in the compressively stressed film. Received: 14 September 2001 / Accepted: 21 January 2002 / Published online: 5 July 2002 RID="*" ID="*"Corresponding author. Fax: +86-21/6598-6323, E-mail: ygwu@mail.tongji.edu.cn     

Extended defects in shallow implants

Extended defects are often found after ion implantation and annealing of silicon and they are known to affect dopant diffusion. The article reviews the structure and energetics of the most often found extended defects and describes the mechanisms by which all these defects grow in size and transform during annealing. Defects grow by interchanging the Si atoms they are composed of and thus maintain large supersaturations of free Si interstitials in the region. A model has been developped to describe such an evolution in presence of a free surface. It is shown that after low energy implantation, the surface of the wafer may recombine large amounts of these free Si interstitials, driving defects into dissolution before transformation into more stable forms. Received: 21 August 2002 / Accepted: 21 August 2002 / Published online: 12 February 2003 RID="*" ID="*"Corresponding author. Fax: +33-56/2257-999, E-mail: claverie@cemes.fr     

13CO2/12CO2 isotopic ratio measurements using a difference frequency-based sensor operating at 4.35 micrometers

A portable modular gas sensor for measuring the ¹³C/¹²C isotopic ratio in CO₂ with a precision of 0.8‰(±1σ) was developed for volcanic gas emission studies. This sensor employed a difference frequency generation (DFG)-based spectroscopic source operating at 4.35 μm (∼2300 cm^-1) in combination with a dual-chamber gas absorption cell. Direct absorption spectroscopy using this specially designed cell permitted rapid comparisons of isotopic ratios of a gas sample and a reference standard for appropriately selected CO₂ absorption lines. Special attention was given to minimizing undesirable precision degrading effects, in particular temperature and pressure fluctuations. Received: 16 April 2002 / Revised version: 28 May 2002 / Published online: 21 August 2002 RID="*" ID="*"Corresponding author. Fax: +1-713/5245237, E-mail: fkt@rice.edu     

Chemical sensing with pulsed QC-DFB lasers operating at 15.6 micrometers

Pulsed thermoelectrically cooled QC-DFB lasers operating at 15.6 μm were characterized for spectroscopic gas sensing applications. A new method for wavelength scanning based on repetition rate modulation was developed. A non-wavelength-selective pyroelectric detector was incorporated in the sensor configuration giving the advantage of room-temperature operation and low cost. Absorption lines of CO₂ and H₂O were observed in ambient air, providing information about the concentration of these species. Received: 29 April 2002 / Published online: 21 August 2002 RID="*" ID="*"Corresponding author. Fax: +1-713/348-5686, E-mail: akoster@rice.edu     

Electromagnetically induced transparency mediated by phonons in strongly coupled exciton–phonon systems

It is shown theoretically that electromagnetically induced transparency (EIT) due to strong exciton–phonon coupling can occur in strongly coupled exciton–phonon systems such as polymers and organic semiconductors and lead to ultra-slow light effects. The results indicate that the strong coupling of excitons and phonons is important, but the exciton– exciton interaction plays a small role in the generation of the EIT. Numerical results for polydiacetylene–toluene sulfonate are also presented. This EIT in a solid-state medium might be utilized for efficient multiwave mixing and quantum nondemolition measurements, as well as for novel acousto-optical devices. Received: 21 August 2002 / Published online: 20 December 2002 RID="*" ID="*"Corresponding author. E-mail: zhukadi@yahoo.com     

Handheld, battery-powered near-IR TDL sensor for stand-off detection of gas and vapor plumes

A handheld, battery-powered tunable-diode-laser sensor platform is described. The sensor is based on frequency modulation (FM) spectroscopy using near-IR diode lasers and passive topographic backscatter from common environmental targets such as buildings, ground and foliage. A specific application to the detection of methane using a 1.65-micron diode laser is described in detail, showing a detection sensitivity sufficient to identify typical leaks from buried residential natural gas service lines at stand-off distances up to 30 m. Signal and noise sources are analyzed in detail, along with laboratory and field-test data, including known service leaks. Received: 15 April 2002 / Revised version: 4 June 2002 / Published online: 21 August 2002 RID="*" ID="*"Corresponding author. Fax: +1-978/689-3232, E-mail: allen@psicorp.com     

Pulsed laser deposition and in situ diagnostics of the process applied to shape-memory alloys

NiTi films deposited by pulsed laser ablation on Si/SiO₂ are shown to exhibit structural and functional properties related to the shape-memory effect. Film characterization suggests that relevant temperatures for the solid-to-solid transformation responsible for the shape memory are in substantial agreement with those of the bulk target material, demonstrating a good congruency of the deposition process. Besides the technological interest for this class of thin films, our findings point out the suitability of laser ablation for metal alloy deposition. An investigation based on in situ ion-mass spectroscopy and covariance mapping analysis allows us to determine the main vapor-phase processes leading to the formation of stoichiometric clusters expected to play a relevant role in assisting the growth of NiTi thin films. Received: 6 August 2001 / Accepted: 11 April 2002 / Published online: 4 November 2002 RID="*" ID="*"Corresponding author. Fax: +39-50/2214-333, E-mail: fuso@df.unipi.it     

Crystal structure, morphology, depth profile of elements and mid-infrared optical constants of ‘mild’ lead telluride film

The characterizations of a so-called ‘mild’ PbTe layer thermal-evaporated from an excess of Te (<1 mol.%) evaporable materials are reported. The results reveal that the film obtained is polycrystalline and has a single-phase NaCl-type PbTe crystal structure. It is also demonstrated that the film has a homogeneous surface morphology and a high degree of homogeneous distribution of Te-rich components along the layer. The study of mid-infrared optical constants of a surface-polished film indicates that the influence of surface scattering on optical properties is very small. Received: 3 July 2002 / Accepted: 7 August 2002 / Published online: 4 December 2002 RID="*" ID="*"Corresponding author. Fax: +86-21/6516-9946, E-mail: lbincome@yahoo.com     

Sensors for scanning probe microscopy

Scanning probe microscopy is still suffering from reproducible fabrication of the corresponding sensors for mechanical, electrical, optical, thermal and chemical material characterisation with highest lateral and/or time resolution. For batch-fabrication techniques lithographic, dry etching and material problems have to be solved. Using such techniques, several types of cantilevers and tips including piezoresistive detection systems have been demonstrated world-wide for the first time. Only an overview is given here. Received: 2 September 2002 / Accepted: 6 November 2002 / Published online: 5 March 2003 RID="*" ID="*"Corresponding author. Fax: +49-561/804-4136, E-mail: kassing@physik.uni-kassel.de     

FIB processing of silicon in the nanoscale regime

We have investigated the impact of shrinking feature sizes on the sputter efficiency of focused ion beams on crystalline silicon. On the basis of this analysis, we have demonstrated the main competitive mechanisms determining the complex response of silicon. Low-dose irradiation with a 50-keV Ga⁺ beam in the range from 10¹³ ions/cm² to 10¹⁵ ions/cm² produced pronounced swelling of the silicon due to amorphization. Higher doses led to material removal with an efficiency of about 2.5 atoms/ion. A sputter yield promoting the self-focusing effect combined with the sputter rate increase at oblique angles, and an opposing dose-deficiency effect, determined the complex characteristics of nanoscale trench milling. Shrinking critical dimensions led to higher sputter yields, attributable to self-focusing effects of incident ions becoming dominant at aspect ratios in the region of unity. At aspect ratios beyond unity, re-deposition was the dominant effect. Received: 17 June 2002 / Accepted: 26 July 2002 / Published online: 4 December 2002 RID="*" ID="*"Corresponding author. Fax: +43-1/588-0136-291, E-mail: alois.lugstein@tuwien.ac.at     

Study on the orientation of silver films by ion-beam assisted deposition

Low energy ion beam assisted deposition (IBAD) was employed to prepare Ag films on Mo/Si (100) substrate. It was found that Ag films deposited by sputtering method without ion beam bombardment were preferred (111) orientation. When the depositing film was simultaneously bombardment by Ar⁺ beam perpendicular to the film surface at ion/atom arrival ratio of 0.18, the prepared films exhibited weak (111) and (200) mixed orientations. When the direction of Ar⁺ beam was off-normal direction of the film surface, Ag films showed highly preferred (111) orientation. Monte Carlo method was used to calculate the sputtering yields of Ar⁺ ions at various incident and azimuth angles. The effects of channeling and surface free energy on the crystallographic orientation of Ag films were discussed.     

Self-organized semiconductor surfaces as templates for nanostructured magnetic thin films

Spontaneous pattern formation during epitaxial growth or ion erosion of semiconductor wafers offers an elegant route towards large-area nanostructured surfaces. In homoepitaxy, kinetics may result in rather uniform three-dimensional islands. In the case of semiconductor heteroepitaxy, strain relief leads to the formation of nanofaceted three-dimensional crystallites, which may self-organize into quasiperiodic arrays. By tuning substrate miscut and film thickness, or growing superlattices, a variety of patterns with different symmetries can be obtained, as will be summarized for the model system of SiGe on Si(001). Since these self-organized nanostructure arrays cover the entire wafer on which they are grown, they can serve as large-area nanopatterned substrates for subsequent deposition of magnetic thin films. It will be demonstrated that such templates allow the study of correlations between magnetic and chemical interfacial roughness, as well as the influence of pattern symmetry on the magnetic anisotropy of thin Co films. Furthermore, shadow deposition of magnetic material onto specially faceted nanostructure arrays allows the fabrication of nanomagnet arrays and the study of their magnetic properties. Received: 31 July 2002 / Accepted: 2 October 2002 / Published online: 5 February 2003 RID="*" ID="*"Corresponding author. Fax: +43-3842/402-760, E-mail: teichert@unileoben.ac.at     

Ion-beam synthesis and growth mechanism of diamond-like materials

Opposite to most other deposition methods, the dominating nucleation and growth mechanism during ion-beam deposition of energetic ions in the range between 10 eV and 10 keV occurs in a region of a few nanometers below the surface of the growing film. This process is called ‘subplantation’ – emphasizing the implantation of ions into a subsurface region. Ordering and phase formation is a result of the interaction of the deposited ions with the solid state that takes place within the short time scale of femto- and picoseconds. This extreme non-equilibrium process can result in metastable amorphous or crystalline structures. This review will present several examples of the influence of the deposition parameters on the properties of diamond-like materials synthesized using mass-selected ion-beam deposition. Furthermore, several existing models of the deposition process will be presented and critically discussed. Received: 11 November 2002 / Accepted: 12 November 2002 / Published online: 4 April 2003 RID="*" ID="*"Corresponding author. Fax: +49-551/39-4493, E-mail: carsten.ronning@phys.uni-goettingen.de