Anders Michael Jørgensen

Lab-on-a-chip with integrated optical transducers

Taking the next step from individual functional components to higher integrated devices, we present a feasibility study of a lab-on-a-chip system with five different components monolithically integrated on one substrate. These five components represent three main domains of microchip technology: optics, fluidics and electronics. In particular, this device includes an on-chip optically pumped liquid dye laser, waveguides and fluidic channels with passive diffusive mixers, all defined in one layer of SU-8 polymer, as well as embedded photodiodes in the silicon substrate. The dye laser emits light at 576 nm, which is directly coupled into five waveguides that bring the light to five different locations along a fluidic channel for absorbance measurements. The transmitted portion of the light is collected at the other side of this cuvette, again by waveguides, and finally detected by the photodiodes. Electrical read-out is accomplished by integrated metal connectors. To our knowledge, this is the first time that integration of all these components has been demonstrated.

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.

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.

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).

Fully integrated optical systems for lab-on-a-chip applications

The integration of optical transducers is generally considered a key issue in the further development of lab-on-a-chip microsystems. We present a technology for the integration of miniaturized, polymer based lasers, with planar waveguides, microfluidic networks and substrates such as structured silicon. The flexibility of the polymer patterning process, enables fabrication of laser light sources and other optical components such as waveguides, lenses and prisms, in the same lithographic process step on a polymer. The optically functionalised polymer layer can be overlaid on any reasonably flat substrate, such as electrically functionalised Silicon containing photodiodes. This optical and microfluidic overlay, interfaces optically with the substrate through the polymer-substrate contact plane. Two types of integrable laser source devices are demonstrated: microfluidic- and solid polymer dye lasers. Both are based on laser resonators defined solely in the polymer layer. The polymer laser sources are optically pumped with an external laser, and emits light in the chip plane, suitable for coupling into chip waveguides. Integration of the light sources with polymer waveguides, micro-fluidic networks and photodiodes embedded in a Silicon substrate is shown in a device designed for measuring the time resolved absorption of two fluids mixed on-chip. The feasibility of three types of polymers is demonstrated: SU-8, PMMA and a cyclo-olefin co-polymer (COC) -- Topas. SU-8 is a negative tone photoresist, allowing patterning with conventional UV lithography. PMMA and Topas are thermoplasts, which are patterned by nanoimprint lithography (NIL).

High-density multimode integrated polymer optics

90° corner bends and waveguide crossings have been investigated in order to increase the integration density of multimode polymer waveguide devices. Using a platform consisting of polymer waveguide cores surrounded by glass and PMMA (polymethylmetacrylate, a clear plastic) as lower and upper cladding, respectively, and air at the sides, it is shown that these components can increase the integration density by a considerable factor with negligible loss penalty. Waveguide crossings have been studied experimentally, and for crossing angles above 35° the excess loss is below 0.15 dB and no excess loss could be measured for a 90° crossing. 90° corner bends have been designed and measured to have very low losses of 0.2 dB over a wide wavelength span. The influence of the waveguide width on the 90° corner bend loss has been characterized and an optimal width has been identified.

Concave reflective SU-8 photoresist gratings for flat-field integrated spectrometers

On-chip spectrometry will play a leading role in the development of micro-optofluidic systems for analytical chemistry. Integrated spectrometers fabricated using a polymer-on-silicon platform have been designed, fabricated, and characterized. Reflective grating designs have been implemented using a recursive algorithm to calculate the facet positions as described by McGreer [Appl. Opt. 35, 5904 (1996)]. It is shown that the free spectral range, the output focal plane geometry, and the linear dispersion can be selected with a high degree of control independently of the chosen grating order. The polymer-on-silicon platform facilitates integration with microfluidic circuits and cost-efficient batch fabrication.

Integration of waveguides for optical detection in microfabricated analytical devices

Buried optical channel waveguides integrated with a fluidic channel network on a planar microdevice are presented. The waveguides were fabricated using silica-on-silicon technology with the goal to replace bulk optical elements and facilitate various optical detection techniques for miniaturized total analysis systems or lab-on-a-chip systems. Waveguide structures with core layers doped with germanium were employed for fluorescence measurements, while waveguides with nitrogen- only doped core layers were used for absorbance measurements. By the elimination of germanium oxygen deficiency centers transmission of light down to 210nm was possible, allowing absorance measurements in the mid and far UV region (210 to 280nm), which is the region where a large number of different molecules absorb light. Robust, alignment-free microdevices, which can easily be hooked up to a number of light sources and detectors were used for fluorescence measurements of two dyes, fluorescein and Bodipy, and absorbance measurements of a stres-reducing drug, propranolol. The lowest detected concentrations were 250pM for fluorescein, 100nM for Bodipy and 12(mu) M for propranolol.

Optimization of SU-8 processing for integrated optics

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.

Novel light trapping scheme for thin crystalline cells utilizing deep structures on both wafer sides [solar cells]

A new light trapping structure is presented with trapping capabilities comparable to or better than those of the perpendicular grooves structure. The new structure traps a larger fraction of rays for 8-80 passes than the perpendicular grooves structure. The average path length enhancement is about 62 times the average thickness. The structure consists of deep (-200 /spl mu/m) inverted pyramids on the front side and deep (-200 /spl mu/m) truncated pyramids with eight sides on the back. The structure is realized in crystalline silicon by wet chemical etching using potassium hydroxide (KOH) and isopropanol (IPA). A process for creating thin solar cells with this light trapping scheme is described. The process includes only two main photolithographic steps and features a self-aligned front metallization. The process uses 250 /spl mu/m wafers to create cells that on average are about 70 /spl mu/m thick.