In this paper we report on an integrated spectrometer device, fabricated in epoxy resist (SU-8) on silicon, designed for
Raman spectroscopy and direct coupling to a CCD element. Furthermore a nanostructured surface is prepared on a gold
coated silicon chip to enhance the Raman signal. We show examples of low resolution Surface Enhanced Raman
Spectra (SERS) recorded with this chip and provide an outlook on the future possibilities. Traditional optical detection in
Lab-on-Chip devices often requires sample pretreatment including chemical reactions in order to identify and detect a
certain substance (e.g. attachment of a fluorescent marker). The basic idea in bringing Raman spectroscopy to the chip is
to avoid these chemical reactions and directly enable identification of the substance by its Raman spectrum.
Two different methods were used to prepare the nanostructured surfaces. The first method is based on an aqueous
suspension of gold nanoparticles and polystyrene beads deposited on a gold surface. The suspension was dried and the
polystyrene beads were removed using an appropriate solvent (methane dichloride). The second approach includes gold
coated random silicon nanostructures so-called "black silicon". The surfaces were characterized using a commercial
Raman spectrometer and the enhancement factor was found to be strongly dependant on the concentration on the sample
The surface was impregnated with a droplet (10 μl, 100 μM) of Rhodamin 6G and Nileblue respectively. Using the on-chip
spectrometer we have recorded surface enhanced Raman spectra of Nileblue and Rhodamin 6G respectively. The
results show that these systems are suitable for low cost extremely compact Raman sensors with possible applications
reaching from process monitoring to homeland security and point of care devices.
Lab-on-chip systems become increasingly more relevant for biochemical analyses. Here is presented a concept for realizing a small footprint chip by combining fluorescence detection and on-chip spectrometry. The chip is to be fabricated using a single mask process based on the negative photoresist SU-8. The various subcomponents are discussed; in particular a spectrometer is presented and interfaced to a linear CCD. The integrated spectrometer combined with the CCD displays a resolving power of 175 for HeNe laser light. The fluidic system is a simple passive microfluidic network which can withstand a pressure in excess of 22 kPa without leakage as long as the sidewall is 10 μm or thicker.
In order to create high-performance integrated optical components based on polymers, such as on-chip spectrometers for lab-on-a-chip, significant process optimization is needed. Here is reported on the results of investigations concerning two aspects of processing of 40 μm thick coatings of the negative photoresist SU-8: 1) development of a process to remove the edge bead after spin coating, in order to reduce proximity effects in the exposure process, and 2) an investigation of parameters in the baking and exposure steps in order to optimize the lithographic resolution. Both aspects were investigated through design of experiment (DOE) and related statistical analysis. The first DOE investigated the significance of eight process parameters in solvent based edge bead removal (EBR), and involved 51 experiments. The optimized process based on the experimental series reduced the edge bead from approximately 30 μm to less than 1 μm, in effect eliminating it. The second DOE covered six parameters; two in the soft bake step, the exposure time, and three in the post-exposure bake. This DOE contained 64 experiments and resulted in significant resolution improvement. Because of the optimization the trench resolution was improved from a starting point of 6 μm to 2.5 μm, and the ridge resolution improved from 7 μm to 5 μm. As a final outcome the best procedure also results in crack-free films which do not delaminate.
More than 80% of all lab-on-a-chip systems rely on optical detection. In most cases this is done by external bulk optical elements. We present an approach where advanced multimode optical elements are integrated with a microfluidic system. In order to ease integration of the optical circuitry, the waveguide height and width are adapted to the dimensions of the microfluidic channels. Typical dimensions for the multimode waveguides are 40 μm x 40 μm. The integrated optical elements include tapers, waveguide crossings, and spectrometers. The devices are designed, simulated and subsequently fabricated in polymer on a silicon substrate. A glass lid bonded to the polymer layer seals the microfluidic channels and provides a top cladding for the waveguide circuitry. Arrays of specially designed components are evaluated to extract precise basic parameters like coupling and propagation loss. To increase compactness of the waveguide circuitry waveguide crossings with different angles are evaluated. It is found the angles down to 25° between the crossing waveguides show little (< 0.25 dB) excess loss. Integrated spectrometers using a reflective, concave echelle grating are fabricated and evaluated. It is shown that spectral range, resolution and linear dispersion of such miniaturized devices can be adapted to the needs of micro total analysis systems (μTAS).