B. Hadam

Fabrication of nanostructures using a UV-based imprint technique

Abstract We present a modified NIL-technique for the fabrication of nm-structures. The approach is based on the photopolymerization of special resits through a quartz mold. The fabrication sequence for the molds as well as requirements and properties of new resist materials are presented. The resists are adapted to the process, making a low pressure and room temperature printing and an easy detachment of the mold possible. Special modification guarantees suitable etching rates and selectivities. With this technique 80% of the mold area was printed with a pressure of only 0.8 bar. Dot arrays with a single dot diameter of 80nm have been printed successfully.

Multiple imprinting in UV-based nanoimprint lithography: related material issues

We will present resist and mold modifications which affect the adhesion forces at the mold/resist interface to assure multiple imprinting in a UV-based stepping process. A specifically modified resist is used in combination with an anti-adhesion layer on the quartz mold surface. The resist was modified with a fluorine-based additive, which migrates to the surface during spin-on processes creating a low energy surface. Auger spectroscopy clearly shows the enhancement of the fluorine at the surface of the resist. The anti-adhesion layer on the mold was characterized by electron spectroscopy for micro-analysis (ESMA) to determine its monolayer characteristic and uniformity. The surface energy of both the modified resist and the mold was determined as well. To demonstrate the success of these modifications, multiple imprints (up to 50 times) of different micro- and nanostructures were performed with one mold. After printing, the structures were transferred into silicon using conventional RIE processes.

Characterization and application of a UV-based imprint technique

We have investigated a UV-based nanoimprint technique with regard to its potential for large area applications. One aspect is the investigation of the residual resist thickness hr, which depends on the geometry of the mold. If the geometry of structures to be printed varies strongly across the mold, fluctuations of the residual resist thickness and incomplete filling of the structures can occur. As a consequence, best results will be obtained with periodic structures and periodically arranged structures as is demonstrated with imprinted metal–semiconductor–metal (MSM) patterns.

SiGe wires and dots grown by local epitaxy

Abstract SiGe quantum wells with finite lateral size have been fabricated using local molecular beam epitaxy through shadowing masks with dimensions down to 200 nm. With this growth technique, mesas of high crystalline quality with sidewalls solely determined by the growth conditions can be achieved and it is possible to grow SiGe quantum wells completely embedded in Si. This greatly reduces nonradiative recombination rates compared to other methods of lateral structuring and leads to the observation of clear excitonic emission even from the smallest SiGe wires. Measured absolute intensities of the SiGe related signals increase by about a factor of two when decreasing the width of the wires from 1 μm to 200 nm. When accounting for the areal coverage of the investigated spot with SiGe, the so normalized intensities strongly increase with decreasing window size and exceed the reference signal by about one order of magnitude for the smallest structures. With decreasing mesa size, a pronounced blue shift is observed which is dependent on the size of the growth window and on the growth temperature. Lateral diffusion effects of Ge on the surface during growth will be discussed in this context.

Metal–oxide–semiconductor-compatible silicon based single electron transistor using bonded and etched back silicon on insulator material

We present a fabrication method for a single electron transistor in silicon. The process is based on bonded and etched back silicon on insulator material with a very thin silicon top layer. Tunnel junctions are realized by electron beam lithography in combination with a two layer resistsystem. The pattern is transferred by anisotropic reactive ion etching and the lateral dimensions are reduced further by thermal oxidation. The process technology is, apart from the e-beam lithography, fully metal–oxide–semiconductor compatible. We electrically characterized samples with four tunnel junctions in series. At 6 K the I/U characteristics reveals a Coulomb blockade as well as a Coulomb staircase, which can be attributed to the asymmetry in the system. Additionally, the temperature dependence and the variation of the background electrostatic potential of the islands were investigated.

Electron beam lithography and ion implantation techniques for fabrication of high-T c Josephson junctions

Abstract Established semiconductor process technologies are demonstrated to be suitable for the fabrication of high temperature superconductor (HTS) Josephson junctions. Single YBCO bridges were modified by local oxygen ion irradiation through a narrow slit in a PMMA mask which was formed by electron beam lithography. The influence of slit dimension and irradiation dose was investigated. The critical current and normal resistance ot the modified microbridges can be controlled by these two parameters. Proximity coupling across the modified region is observed up to a slit width of 250 nm. When exposed to microwave irradiation the microbridges exhibited Shapiro steps. In dc SQUIDs a voltage modulation as a function of an applied magnetic flux is observed.

Direct patterning of single electron tunneling transistors by high resolution electron beam lithography on highly doped molecular beam epitaxy grown silicon films

We present a fabrication method for a single electron tunneling transistor (SETT) in silicon. The process is based on bonded and etched back silicon on insulator material with a 40 nm thick highly n-doped Si layer grown by molecular beam epitaxy. The nanometer structure of the SETT is defined by electron beam lithography in combination with a two-layer resist system. The pattern is transferred by anisotropic reactive ion etching. The devices are passivated by low temperature remote plasma enhanced chemical vapor deposition of high quality silicondioxide. An extended region of low conductivity is observed even at T=130 K in the current–voltage characteristics, outside of which a strong Coulomb staircase is visible. The Coulomb blockade is significantly affected by the applied gate voltage. Coulomb oscillations of the blockade width with gate potential are observed.

Fabrication and characterisation of SiGe based in-plane-gate transistors

Abstract We present an in-situ technology for fabrication of barrier structures in modulation-doped Si SiGe in-plane-gate (IPG) transistors. A special multilayer-resist system is developed for pattern transfer by electron-beam lithography (EBL) and anisotropic SF 6 O 2 dry etching. Barriers are realized by etch-trenches cutting the two dimensional electron gas (2DEG). The trenches are filled up with a low temperature remote plasma enhanced chemical vapour deposition (RPECVD) of silicondioxide (SiO2). Dry-etching and passivation are done in-situ to avoid contamination. IPG transistors with different geometric dimensions have been fabricated and electrically characterised. Transistor operation is demonstrated up to T=77 K. The breakdown voltage and the depletion length of the devices are estimated. The obtained data indicate the advantage of the presented in-situ technology in comparison to other fabrication techniques.

Local epitaxy of Si/SiGe wires and dots

Local epitaxy through shadowing masks has been used to fabricate well-passivated SiGe quantum wells with lateral dimensions from 15 μm down to 200 nm. The resulting SiGe mesas are investigated with conventional photoluminescence (PL) and spatially resolved PL. Using spatially resolved PL, the luminescence of isolated single SiGe dots is observed for the first time. For mesa sizes below 1 μm, the normalized intensity increases with decreasing size and exceeds the reference signal by more than one order of magnitude for mesas grown through 200 nm wide windows. All investigated mesas show a pronounced blue shift compared to the reference areas. While the dependence of the blue shift on the mesa size is consistent with a very simple surface diffusion model, the observed dependence on the growth temperature is more complicated and not quite understood. Finally, the luminescence of a single 1 μm wide SiGe wire in dependence of the detected wire length after local excitation is used to estimate the exciton density distribution along the wire. A simple exponential decay fits the experimental data well with a decay length of about 10 μm.

Investigation of Si/SiGe heterostructures patterned by reactive ion etching

Abstract We present a modified pattern technique for fabrication of nanometer structures in the Si SiGe heterosystem. A special multilayer-resist system is developed for pattern transfer by electron-beam lithography and anisotropic SF 6 O 2 dry etching. Photoluminescence measurements are carried out on homogeneously etched samples to determine the influence of the RIE process on the optical properties. Etching induced damages are reduced by a low-temperature post-annealing step. Additionally, surface contaminations are investigated using laser desorption mass spectrometry. SiGe wires with lateral widths from 4 μm down to 25 nm have been fabricated, but photoluminescence has been observed for structures down to 600 nm lateral width, only. Further improvement has been obtained by sidewall-passivation of the nanostructures with a low temperature plasma enhanced-chemical-vapour-deposited oxide. Up to now, clear excitonic emission is detected for wires as small as 250 nm.