Sub-μm Structures Research Articles

Laboratory High-Contrast X-ray Microscopy of Copper Nanostructures Enabled by a Liquid-Metal-Jet X-ray Source.

High-resolution imaging of Cu/low-k on-chip interconnect stacks in advanced microelectronic products is demonstrated using full-field transmission X-ray microscopy (TXM). The comparison of two lens-based laboratory X-ray microscopes that are operated at two different photon energies, 8.0 keV and 9.2 keV, shows a contrast enhancement for imaging of copper nanostructures embedded in insulating organosilicate glass of a factor of 5 if 9.2 keV photons are used. Photons with this energy (Ga-Kα radiation) are generated from a Ga-containing target of a laboratory X-ray source applying the liquid-metal-jet technology. The 5 times higher contrast compared to the use of Cu-Kα radiation (8.0 keV photon energy) from a rotating anode X-ray source is caused by the fact that the energy of the Ga-Kα emission line is slightly higher than that of the Cu-K absorption edge (9.0 keV photon energy). The use of Ga-Kα radiation is of particular advantage for imaging of copper interconnects with dimensions from several 100 nm down to several 10 nm in a Cu/SiO2 or Cu/low-k backend-of-line stack. Physical failure analysis and reliability engineering in the semiconductor industry will benefit from high-contrast X-ray images of sub-μm copper structures in microchips.

Nanometer Sized Direct Laser-Induced Gold Printing for Precise 2D-Electronic Device Fabrication.

Flexible electronics manufacturing technologies are essential and highly favored for future integrated photonic and electronic devices. Direct laser induced writing (DIW) of metals has shown potential as a fast and highly variable method in adaptable electronics. However, most of the DIW procedures use silver structures, which tend to oxidize and are limited to the micrometer regime. Here, a DIW technique is introduced that not only enables electrical gold wiring of 2D van-der-Waals materials with sub-µm structures and 100nm interspacing resolution but is also capable of fabricating photo switches and field effect transistors on various rigid and elastic materials. Light sensitive metalloid Au32 -nanoclusters serve as the ink that allows for low-power cw-laser exposure without further post-treatment. With a simple lift-off procedure, the unexposed ink can be removed. The technique realizes ultrafast, high resolution, and high precision production of integrated electronics and may pave the way for personalized circuits even printed on curved surfaces.

Nonlinear optical feedback for nano- and micropatterning of silicon surface under femtosecond laser irradiation

Silicon based micro- and nanophotonics yielding plenty of applications from black silicon for photovoltaics to all-dielectric nanoantennas for sensing and nonlinear optics requires advanced methods of fabrication. Here we provide a detailed investigation of femtosecond laser fabrication of silicon based miro- and nanostructures formed via self-organization processes revealing a strong influence of nonlinear optical feedback during their growth under multipulse irradiation. Our results are supported by the experimental study of the effects of laser wavelength, fluence, exposure time, and ambient gas pressure. We demonstrate that both sub-µm and µm-scale structures have similar principles of formation, which paves the way to controllable and low-cost fabrication of advanced optoelectronics devices with multilevel patterning of functional elements.

Replication fidelity assessment of large area sub-μm structured polymer surfaces using scatterometry

The present study addresses one of the key challenges in the product quality control of transparent structured polymer substrates, the replication fidelity of sub-μm structures over a large area. Additionally the work contributes to the development of new techniques focused on in-line characterization of large nanostructured surfaces using scatterometry. In particular an approach to quantify the replication fidelity of high volume manufacturing processes such as polymer injection moulding is presented. Both periodic channels and semi-spherical structures were fabricated on nickel shims used for later injection moulding of Cyclic-olefin-copolymer (COC) substrate were the sub-μm features where ultimately transferred. The scatterometry system was validated using calibrated atomic force microscopy measurements and a model based on scalar diffraction theory employed to calculate the expected angular distribution of the reflected and the transmitted intensity for the nickel surfaces and structured COC and, respectively.

Single-crystalline Bi2Sr2CaCu2O8+x detectors for direct detection of microwave radiation

We test radiation detectors made from single-crystalline Bi2Sr2CaCu2O8+x flakes put on oxidized Si substrates. The 100-nm-thick flakes are lithographically patterned into 4×12 μm2 large rectangles embedded in thin-film log-spiral antennas. The SiO2 layer weakens the thermal link between the flakes and the bath. Two modes of radiation detection have been observed. For a bolometric type of sensors a responsivity of ∼300 V/W and a noise equivalent power of 30 nW/Hz has been deduced at 70 K. Much more sensitive is the non-bolometric device showing characteristics similar to a Golay-type detector while being at least a thousand times faster. Making smaller (sub-μm) structures is expected to significantly improve the performance of these devices and makes them very competitive among other microwave and terahertz detectors.

Structuring of glass fibre surfaces by laser-induced front side etching

The fabrication of sub-μm structures on glass fibre surfaces poses a big challenge for the laser processing. However, the laser-induced front side etching (LIFE) method has a great potential for the fast, nm-precision, and cost-effective production of surface structures. LIFE is a method for laser etching of transparent materials using thin absorber layers with a high absorption coefficient like metal layers. The LIFE process of the front surface of a fused silica wafer as well as of a glass fibre is studied in dependence on the laser parameters. A KrF excimer laser with a wavelength of 248nm and a pulse duration of 25ns was used. The resultant structures were analysed with microscopic methods (white light interferometry, scanning electron microscopy (SEM)). The analysing of the surface structures presented that the LIFE methods allow the fabrication of well-defined periodic sub-μm structures. Furthermore, the structuring process was simulated by a thermodynamic equation including an approach of the laser–plasma interaction. The theoretically predicted results presented a good agreement with the experimental results.

Quasi-periodic self-assembled sub-micrometer ferroelectric bulk domain gratings in Rb-doped KTiOPO4

We present a simple technique for fabricating quasi-periodic bulk sub-μm ferroelectric domain gratings in Rb-doped KTiOPO4 (RKTP) based on self-organized ferroelectric domain formation. One-dimensional ferroelectric domain structures, with an average periodicity of 650 ± 200 nm and extending throughout 1 mm thick crystals, are obtained by etching and subsequent electric field poling using planar electrodes. The sub-μm structures in RKTP were used to demonstrate 5th order non-collinear quasi-phase matched backward second harmonic generation.

Direct bonding for the encapsulation of transverse optical gratings

► Successful direct bonding of SiO 2 binary gratings in transverse orientation. ► Gap opening method reveals different bond strengths at similar grating periods. ► Different bond strengths explained from stress concentrations at the gap tips. ► Direct bonding verified for joining sub-μm structures of fused silica. Direct bonding is a suitable technology for joining glass materials without affecting transmission and encapsulated gratings are of interest in laser and spectroscopic applications. We report on direct bonding of fused silica wafers with gratings at their surfaces for diffractive optical applications in the visible and near infra-red spectral range. Binary gratings with periods of 1000 nm and 1500 nm were generated in 6 in. fused silica wafers by e-beam lithography and low pressure reactive ion etching. The structured surfaces were aligned to each other at ambient conditions after extended wet cleaning and low pressure plasma activation. Successful bonding was achieved under compressive pressure at low temperatures in a vacuum environment. Even for transverse grating orientations, virtually “defect free” bonding across the whole grating area was obtained. Bonding strengths were determined by the gap opening method and revealed different values according to the direction of opening relative to the grating period.

Sub-μm structured lotus surfaces manufacturing

Sub-μm structured surfaces allow modifying the behavior of polymer films or components. Especially in micro-fluidics a lotus-like characteristic is requested for many applications. Structure details with a high aspect ratio are necessary to decouple the bottom and the top of the functional layer. Unlike to stochastic methods, patterning with a LIGA-mold insert it is possible to structure surfaces very uniformly or even with controlled variations (e.g., with gradients). In this paper we present the process chain to realize polymer sub-μm structures with minimum lateral feature size of 400 nm and up to 4 μm high.

Selective deposition of functionalized nano-objects by nanoxerography

Guided assembly of nanoscale objects on solid substrates via electrostatic attraction represents a simple and versatile process for the maskless fabrication of sub-μm structures and arrays of single nano-objects. To this end, patterns of local surface charges are written into an electret layer by applying voltage pulses to a conductive atomic force microscope tip. These charge patterns act as templates for the deposition of nanoscale objects in the subsequent development step. Here we describe the direct deposition of surface functionalized polystyrene beads and multiwalled carbon nanotubes from alcohol-based suspensions. Carboxyl groups lead to negative surface charges on suspended nano-objects resulting in Coulomb attraction to the positively charged templates. Assays using 100 nm diameter functionalized polystyrene spheres demonstrate that this nanoxerography method yields dense monolayers with excellent pattern definition and also allows for the precise arraying of single beads. Carboxylated carbon nanotubes deposit as dense layer of mainly single nanotubes on the predefined charge patterns.

Periodic Poling of KTiOPO4: From Micrometer to Sub-Micrometer Domain Gratings

Some novel techniques to do periodic poling of KTP are described. Emphasis on ways to avoid domain broadening and merging of domains is presented with reference to the original properties of flux grown KTP. For sub-μm structures E-beam lithography and deep UV-laser lithography were used to pattern the samples, and chemical process to promote selective poling to obtain high quality domain gratings in up to 1 mm thick samples.

Focused ion beam machined nanostructures depth profiled by macrochannelling ion beam analysis

High aspect ratio sub-μm periodic structures fabricated by focused ion beam (FIB) lithography have been characterised by Rutherford backscattering spectrometry (RBS) using the macrochannelling technique. The technique overcomes the limitations of complementary techniques such as scanning electron microscopy (SEM) and atomic force microscopy (AFM), which can provide images with sub-μm resolution of just the surface features and not of the deep sub-surface structures, without destructive cross sectioning of the sample. Here RBS macrochannelling with a 2MeV He+ ion beam is used to analyse a diffraction grating fabricated by FIB milling an array of 100nm wide trenches in a 300nm thick Ag film on a Si substrate. Using the surface structure imaged by SEM and AFM as a starting point, a numerical model for the RBS spectrum from the grating is fitted to the experimental spectrum as a function of the sub-surface structure. This process allows the width of the trenches to be determined as a function of depth even though the lateral structure is not resolved by the ion beam.

Nonresonant Laser–SNMS and TOF–SIMS analysis of sub-μm structures

We determined and compared the capabilities of TOF–SIMS and Laser–SNMS for chemical mapping of sub-μm structures by investigating different inorganic and organic samples. For our investigations we used a combined TOF–SIMS/Laser–SNMS instrument, equipped with a focused (<80 nm) Ga +-LMIS and an excimer laser for nonresonant postionization of sputtered neutrals. For 16 monoelemental metal samples we determined useful yields up to 10 −2 (Laser–SNMS) and 10 −3 (TOF–SIMS), respectively. The high yields allow chemical characterization of metal oxide particles, such as Al 2O 3, Fe 3O 4 and CeO 2 with diameters well below the spot size of the primary ion beam, as well as the detection and identification of metal loads on nanotips and semiquantitative elemental mapping of catalysts. Compared with TOF–SIMS, Laser–SNMS features a number of advantages for elemental analysis: higher and more uniform yields, i.e. sensitivities, ease of quantification because of a drastically reduced matrix effect, and compatibility with extreme surface topographies, since sputtered neutrals are not affected by distortions of the extraction field. Laser–SNMS and TOF–SIMS results for phase separated phospholipid/protein LB monolayers on Au substrates demonstrate the capabilities of these techniques for molecular surface mapping.

Versatile Nanodeposition of Dielectrics and Metals by Non-contact Direct-Write Technology

ABSTRACTDirect-write techniques allow processing in the nanometer range and have become powerful methods for rapid prototyping of microelectronic circuits and micro-electro-mechanical systems (MEMS). Chemical reactions are initiated by a focused beam leading to deposition of solid material on literally any surface. We have used this method to deposit metals such as tungsten and dielectrics such as silicon oxide using a focused ion beam (FIB) with 10 to 50 kV acceleration voltage. Controlled guidance of the beam allows deposition of both metallic and dielectric material with features in the 100 nm range. The deposition of separate structures of metallic and dielectric material deposited next to each other is shown on samples of different roughness. 3-dimensional exemplary prototypes in the sub-μm range and multilayer structures demonstrate the versatility of this method for prototyping and mix-and-match approaches with commercial semiconductor devices. A characterization of the deposited material was performed to clarify chemical composition and surface morphology of deposited structures. The deposition parameters were found to influence the chemical composition and electronic properties of the material. Direct-write deposition of dielectrics and metals by FIB allows fabrication of 3-dimensional prototypes with custom-tailored material properties.

Electrical characterization of the SiO 2Si system

The classical capacitance-voltage measurement methods and the conductance technique which are frequently used to analyze the SiO 2Si system, are briefly introduced and critically evaluated. These measurement techniques are based on macroscopic device structures, average structure parameters are determined. Transient and random telegraph signal measurements, and charge pumping are now available as new methods to study the SiO 2Si interface in sub-μm device structures. These techniques permit to analyze single, individual traps in the interface. Traps in small devices are isolated. The capture rates are thermally activated by the Coulomb energy.

Combined ultrahigh vacuum scanning tunneling microscope scanning electron microscope system

We describe a combined ultrahigh vacuum scanning tunneling microscope (STM)–scanning electron microscope (SEM) system, which allows to position the STM tip with respect to the sample within an area of 5 mm×5 mm under SEM control. While the SEM resolution is sufficient to clearly resolve sub-μm structures on the samples, the STM features atomic resolution on semiconductor surfaces. The combination of SEM and STM allows high-resolution studies on inhomogeneous samples in materials research as well as the use for micro- and nanoelectronic device characterization or device modification. The STM performance was checked by atomically resolved imaging of Si(111) (7×7) surfaces. The STM/SEM combination and its application in device characterization is demonstrated by the investigation of vertically grown resonant tunneling diodes on an AlAs/GaAs basis. Due to its performance the system has a high potential for high-resolution imaging in materials research, for novel device characterization and nanoscale structuring or modification of very small devices.

Scalable Fabrication and Optical Characterization of Nm Si Structures

ABSTRACTObservations of efficient room temperature photoluminescence (PL) from porous Si have generated a great deal of interest in the optical properties of nm-scale Si structures. The stochastic character of porous-Si fabrication results in a distribution of crystal sizes and shapes. We report on a scalable (to large areas) and manufacturable (to high volumes) fabrication technology for uniform, nm-linewidth Si structures providing an important testbed for controlled studies of these optical properties. Large areas ( ∼ 1 cm2) of extreme sub-μm structures (to ∼ 5 nm) are re-producibly fabricated. Both walls (1-D confinement) and wires (2-D confinement) are reported. The fabrication process includes: interferometric lithography, highly anisotropic KOH etching, and structure dependent oxidation. For the walls, nearly perfect &lt;111&gt; crystal planes form the sidewalls and very high width/depth aspect ratios (&gt; 50) have been achieved. Raman scattering results on the walls demonstrate three regimes: 1) lineshapes and cross sections similar to bulk Si for line widths, W &gt; 200 nm; 2) electromagnetic resonance enhancement of the cross section ( to - 100x) for W from 50-200 nm; and 3) highly asymmetric lineshapes and splittings from W &lt; 30 nm. Photoluminescence is observed for the thinnest samples (W &lt; 10 nm) and is as intense as that observed from porous Si with a spectral linewidth ∼ 50 % smaller than that of porous Si.

Calculation of the influence of electric fields on primary and secondary electrons in electron beam testing of submicron structures

The decreasing structure sizes of new electronic devices make faultless measuring of signals at internal nodes of electronic devices by means of electron beam testing (EBT) difficult. The aim of this paper is to show a way to simulate the influence of electric fields on primary (PE) and secondary (SE) electrons using the finite difference method (FDM). To determine failures in EBT, being an essential for a quantitative investigation of sub-μm structures, simulations and measurements of the PE-beam shift and simulations demonstrating the influence of electric fields on the whole SE-spectrum emitted from the sample will be shown.