Thomas Aarøe Anhøj

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.

Magnetic properties of Fe1−xMnx/Fe nanocomposites

We have prepared nanocomposites of mixtures of ferromagnetic α-Fe and antiferromagnetic γ-Fe50Mn50 nanoparticles, and studied their magnetic and structural properties by magnetization measurements, Mossbauer spectroscopy, and x-ray diffraction. A sample consisting of a 1:1 mixture of the two materials showed enhanced coercivity, but almost negligible exchange bias at room temperature after field cooling from 520 K. However, samples with higher content of γ-Fe50Mn50 showed significant exchange bias. The mechanisms for exchange bias and enhanced coercivity in the system are discussed.

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.

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.

On-chip integrated spectrometer and microfluidic fluorescence set-up

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.