Jörg Hübner

Large Area Fabrication of Leaning Silicon Nanopillars for Surface Enhanced Raman Spectroscopy

Using a simple two step fabrication process substrates with a large and uniform Raman enhancement, based on flexible free standing nanopillars can be manufactured over large areas using readily available silicon processing equipment.

Monolithic integration of optical waveguides for absorbance detection in microfabricated electrophoresis devices.

The fabrication and performance of an electrophoretic separation chip with integrated optical waveguides for absorption detection is presented. The device was fabricated on a silicon substrate by standard microfabrication techniques with the use of two photolithographic mask steps. The waveguides on the device were connected to optical fibers, which enabled alignment free operation due to the absence of free‐space optics. A 750 νm long U‐shaped detection cell was used to facilitate longitudinal absorption detection. To minimize geometrically induced band broadening at the turn in the U‐cell, tapering of the separation channel from a width of 120 down to 30 νm was employed. Electrical insulation was achieved by a 13 νm thermally grown silicon dioxide between the silicon substrate and the channels. The breakdown voltage during operation of the chip was measured to 10.6 kV. A separation of 3.2 νM rhodamine 110, 8 νM 2,7‐dichlorofluorescein, 10 νM fluorescein and 18 νM 5‐carboxyfluorescein was demonstrated on the device using the detection cell for absorption measurements at 488 nm.

Integrated optical measurement system for fluorescence spectroscopy in microfluidic channels

A transportable miniaturized fiber-pigtailed measurement system is presented which allows quantitative fluorescence detection in microliquid handling systems. The microliquid handling chips are made in silica on silicon technology and the optical functionality is monolithically integrated with the microfluidic channel system. This results in inherent stability and photolithographic alignment precision. Permanently attached optical fibers provide a rugged connection to the light source, detection, and data processing unit, which potentially allows field use of such systems. Fluorescence measurements with two dyes, fluorescein, and Bodipy 650/665 X, showed good linear behavior over a wide range of concentrations. Minimally detected concentrations were 250 pM for fluorescein and 100 nM for Bodipy.

Monolithic integration of microfluidic channels and optical waveguides in silica on silicon

Sealing of the flow channel is an important aspect during integration of microfluidic channels and optical waveguides. The uneven topography of many waveguide-fabrication techniques will lead to leakage of the fluid channels. Planarization methods such as chemical mechanical polishing or the etch-back technique are possible, but troublesome. We present a simple but efficient alternative: By means of changing the waveguide layout, bonding pads are formed along the microfluidic channels. With the same height as the waveguide, they effectively prevent leakage and hermetically seal the channels during bonding. Negligible influence on light propagation is found when 10-mum-wide bonding pads are used. Fabricated microsystems with application in absorbance measurements and flow cytometry are presented.

Single-Mode Waveguides With SU-8 Polymer Core and Cladding for MOEMS Applications

Fabrication and optical characterization of single-mode polymeric embedded waveguides are performed. A specific material combination (SU-8 2005 as core and the modified SU-8 mr-L 6050XP as cladding) is chosen in order to obtain a small refractive index difference for single-mode propagation combined with the conventional fabrication method UV lithography to facilitate the integration of different types of optical detection methods on lab-on-a-chip systems. We analyze the behavior of the refractive index and carefully observe how the value of the refractive index can be tailored during processing. We show that we can fabricate waveguides with an index difference in the order of 10-3 , where both the core material and the cladding material are based on SU-8. The refractive index measurements are performed on thin polymeric films, while further optical characterizations are performed on waveguides with a height of 4.5 mum. We study the mode profiles of these waveguides and confirm that only the fundamental mode is excited. We also study the absorption spectra of the material in the wavelength range 800-1600 nm combined with cut-back measurements. We find that the waveguides have a propagation loss of 0.2-3 dB/cm in this wavelength range

The effect of soft bake temperature on the polymerization of SU-8 photoresist

This paper presents the results of an investigation of the influence of soft baking temperature on the lithographic performance of the negative photoresist SU-8. The work was initiated in order to obtain a lithographic resolution suitable for integration of diffractive optical components for near-infrared wavelengths. The study was carried out on 40 µm SU-8 layers on thermally oxidized silicon wafers, a widespread platform for integration of microfluidic systems and waveguides. A series of experiments covering soft bake temperatures in the range 65–115 °C were performed under otherwise identical processing conditions. The influence of the soft bake temperature on polymerization temperature as well as cracking, lithographic resolution and hardness of the resist was investigated. The kinetics of the polymerization process were observed to change with soft bake temperature, leading to changes in sensitivity and contrast of the resist, as well as changes in the material strength of the developed structures. Soft baking at 65 °C proved superior with respect to all the inspected properties, providing a sample showing full resolution of 3.8 µm wide trenches and no stress-related cracking.

Integrated optical readout for miniaturization of cantilever-based sensor system

The authors present the fabrication and characterization of an integrated optical readout scheme based on single-mode waveguides for cantilever-based sensors. The cantilever bending is read out by monitoring changes in the optical intensity of light transmitted through the cantilever that also acts as a waveguide. The complete system is fabricated in the photosensitive polymer SU-8. They show theoretical calculations on the expected sensitivity both when operated in air and liquid and compare these with experimental characterization of the system in air where the cantilever is deflected mechanically. The experimental results compare well with the results obtained from the theoretical calculations.

Ultraviolet transparent silicon oxynitride waveguides for biochemical microsystems

The UV wavelength region is of great interest in absorption spectroscopy, which is employed for chemical analysis, since many organic compounds absorb in only this region. Germanium-doped silica, which is often preferred as the waveguide core material in optical devices for telecommunication, cannot accommodate guidance below 400 nm, owing to the presence of UV-absorbing centers. We show that silicon oxynitride (SiO(x) N(y)) waveguides exhibit very good UV performance. The propagation loss for 24-microm -wide SiO(x)N (y) waveguides was found to be ~1.0dB/cm in the wavelength range 220-550 nm. The applicability of these waveguides was demonstrated in a biochemical microsystem consisting of multimode buried-channel SiO(x)N (y) waveguides that were monolithically integrated with microfluidic channels. Absorption measurements of a beta -blocking agent, propranolol, at 212-215 nm were performed. The detection limit was reached at a concentration of 13microM , with an optical path length of 500microm (signal/noise ratio, 2).

Strong sampled Bragg gratings for WDM applications

Strong sampled Bragg gratings are UV induced in planar low-loss silica waveguides. Using such gratings, multichannel wavelength-division-multiplexing (WDM) components can be manufactured with high precision. An ASE filter and multichannel add/drop multiplexer have been demonstrated and characterized using sampled gratings and optical circulators. The add/drop multiplexer exhibits an intraband crosstalk of -30 dB and an interband crosstalk of -20 dB when used with a 200-GHz channel spacing. Simulations using a matrix method are performed showing that the use of uniform sampled gratings is limited by interband crosstalk to channel spacings of around 100 GHz. Propagation losses induced by the amplitude modulated superstructure of sampled gratings are investigated.

Towards easily reproducible nano-structured SERS substrates

In this paper we present a quick and easy method for producing relatively large areas of substrate that enhance the Raman effect, using standard semiconductor processing techniques such as reactive ion etching of silicon and electron beam metal deposition. As standard cleanroom processes are used, it is possible to narrowly control the parameters of the fabrication process to create silicon nano-pillars with controlled heights and spacing. The silicon nano-pillars are coated by thin films of silver and/or gold to create surfaces that greatly enhance the Raman effect. Surface enhanced Raman scattering (SERS) has numerous applications in chemical sensing, with high sensitivity and fast analysis speed seen as the main advantages, and these novel substrates are believed to be able to make SERS more applicable.