Christian Dais

Displacement Talbot lithography: a new method for high-resolution patterning of large areas

Periodic micro and nano-structures can be lithographically produced using the Talbot effect. However, the limited depth-of-field of the self-images has effectively prevented its practical use, especially for high-resolution structures with periods less than 1 micrometer. In this article we show that by integrating the diffraction field transmitted by a grating mask over a distance of one Talbot period, one can obtain an effective image that is independent of the absolute distance from the mask. In this way high resolution periodic patterns can be printed without the depth-of-field limitation of Talbot self-images. For one-dimensional patterns the image obtained is shown to be related to the convolution of the mask transmission function with itself. This technique, which we call Displacement Talbot Lithography (DTL), enables high-resolution photolithography without the need for complex and expensive projection optics for the production of periodic structures like diffraction gratings or photonic crystals. Experimental results showing the printing of linear gratings and an array of holes on a hexagonal lattice are presented.

Extreme ultraviolet interference lithography at the Paul Scherrer Institut

We review the performance and applications of an extreme ultraviolet interference lithography (EUV-IL) system built at the Swiss Light Source of the Paul Scherrer Institut (Villigen, Switzerland). The interferometer uses fully coherent radiation from an undulator source. 1-D (line/space) and 2-D (dot/hole arrays) patterns are obtained with a transmission-diffraction-grating type of interferometer. Features with sizes in the range from one micrometer down to the 10-nm scale can be printed in a variety of resists. The highest resolution of 11-nm half-pitch line/space patterns obtained with this method represents a current record for photon based lithography. Thanks to the excellent performance of the system in terms of pattern resolution, uniformity, size of the patterned area, and the throughput, the system has been used in numerous applications. Here we demonstrate the versatility and effectiveness of this emerging nanolithography method through a review of some of the applications, namely, fabrication of metallic and magnetic nanodevice components, self-assembly of Si/Ge quantum dots, chemical patterning of self-assembled monolayers (SAM), and radiation grafting of polymers. (c) 2009 Society of Photo-Optical Instrumentation Engineers. [DOI: 10.1117/1.3116559]

Impact of template variations on shape and arrangement of Si∕Ge quantum dot arrays

Templated self-assembly allows the fabrication of quantum dot (QD) arrays for use in nanoelectronic devices. Here, we show the strong dependence of the shape and arrangement of QDs on the template structures. Arrays of etched pits are patterned on Si (100) substrates by extreme ultraviolet interference lithography on which Si∕Ge layers are grown in a molecular beam epitaxy system. Single Ge dome clusters or quantum molecules consisting of four Ge hut clusters are obtained by a change of the pit diameter. Both arrays exhibit a narrow size distribution and exact alignment of the dots. In addition, multiple stacking of these arrays is demonstrated.

Photoluminescence studies of SiGe quantum dot arrays prepared by templated self-assembly

The photoluminescence emission of SiGe quantum dot arrays prepared by templated self-assembly, combining extreme-ultraviolet interference lithography and molecular beam epitaxy, were studied. The PL spectra obtained from areas with ordered dots show a pronounced SiGe-quantum-dot–related signal. The corresponding no-phonon and assisted transversal optical phonon recombinations are well resolved due to the narrow-size distribution of the fabricated quantum dot arrays. Additionally, the dependence of the photoluminescence emission on dot size and Ge concentration is discussed as well as effects of laser power excitation.

Three-dimensional phononic nanocrystal composed of ordered quantum dots

We demonstrated a nanoscaled artificial phononic crystal composed of three-dimensionally ordered quantum dots (QDs) with functional acoustic properties. Femtosecond ultrasonic technique is used to investigate the lattice dynamics of this phononic nanocrystal. The measurement results indicate that three-dimensional ordering and uniformity of the QDs are important factors influencing the observed acoustic resonance at the forbidden bands. For well-arranged QDs, noticeable features of the phononic band gap and the associated phonon cavity mode can be found, while this nanocrystal also serves as an effective acoustic medium, or acoustic meta material, for low-frequency acoustic phonons.

Evolution and stability of ordered SiGe islands grown on patterned Si(100) substrates

SiGe quantum dots are proposed as building blocks for future Si device technology. However, in order to exploit the full potential of SiGe islands, it is necessary to control their positioning and size on a nanometer length. This is achieved by templated self-assembly, which combines substrate patterning and subsequent epitaxy. In this paper we report on the evolution of SiGe islands on patterned substrates under consideration of small template variations and postgrowth annealing. The impact of the structural variations on the optical properties of the islands is investigated by photoluminescence measurements.

SiGe quantum dot crystals with periods down to 35 nm

By combining extreme ultraviolet interference lithography with Si/Ge molecular beam epitaxy, densely packed quantum dot (QD) arrays with lateral periodicities down to 35 nm are realized. The QD arrays are featured by perfect alignment and remarkably narrow size distribution. Also, such small periodicities allow the creation of three-dimensional QD crystals by vertical stacking of Si/Ge layers using very thin Si spacer layers. Simulations show that the distances between adjacent QDs are small enough for coupling of the electron states in lateral as well as vertical directions.

3D SiGe QUANTUM DOT CRYSTALS: STRUCTURAL CHARACTERIZATION AND ELECTRONIC COUPLING

We report on the growth of SiGe quantum dot crystals which are realized by depositing Ge on a two-dimensionally pit-patterned Si substrate and subsequent growth of Si spacer and Ge island layers. Lateral periods of 100 nm are obtained by employing deep UV lithography using synchrotron radiation. The vertical period of the typically 10 period dot superlattices was of the order of 10 nm. Ordering of the islands was investigated by atomic force microscopy as well as by high resolution x-ray diffraction studies. From the quantitative evaluation of the x-ray diffraction data a mean Ge content of about 60% in the quantum dots was obtained and an rms. deviation from ideal lattice sites of about 3 nm was found. A simulation of the eigenenergies based on the nextnano3 simulation package was used to interpret the measured photoluminescence data.

Titania-assisted electron-beam and synchrotron lithography

Novel imaging layer technology for electron-beam and extreme-ultraviolet lithographic processes based upon generation of Pd nanoparticles in the Pd(2+)-loaded TiO(2) films was developed. The electroless metallization of the patterned TiO(2):Pd(2+) films yields both negative and positive nickel images with resolution down to approximately 100 nm.

High aspect ratio silicon structures by Displacement Talbot lithography and Bosch etching

Despite the fact that the resolution of conventional contact/proximity lithography can reach feature sizes down to ~0.5- 0.6 micrometers, the accurate control of the linewidth and uniformity becomes already very challenging for gratings with periods in the range of 1-2 μm. This is particularly relevant for the exposure of large areas and wafers thinner than 300 μm. If the wafer or mask surface is not fully flat due to any kind of defects, such as bowing/warpage or remaining topography of the surface in case of overlay exposures, noticeable linewidth variations or complete failure of lithography step will occur. We utilized the newly developed Displacement Talbot lithography to pattern gratings with equal lines and spaces and periods in the range of 1.0 to 2.4 μm. The exposures in this lithography process do not require contact between the mask and the wafer, which makes it essentially insensitive to surface planarity and enables exposures with very high linewidth uniformity on thin and even slightly deformed wafers. We demonstrated pattern transfer of such exposures into Si substrates by reactive ion etching using the Bosch process. An etching depth of 30 μm or more for the whole range of periods was achieved, which corresponds to very high aspect ratios up to 60:1. The application of the fabricated gratings in phase contrast x-ray imaging is presented.